THE  RESEARCH  LABORATORY,  SCHENECTADY 


OC 


This  volume  is  a  collection  of  papers  describing 
original  researches  which  have  been  made  by  vari- 
ous members  of  the  staff  of  the  Research  Labora- 
tory, of  the  General  Electric  Company,  relating  to 
the  subject  of  X-rays.  They  have  appeared  from 
time  to  time  in  scientific  periodicals,  as  separately 
indicated.  It  has  seemed  wise  to  collect  and  re- 
publish  them  because  of  the  interest  they  might 
have  to  Roentgenologists. 

Permission  to  reprint  them  has  been  duly  received, 
and  we  herewith  express  our  thanks  to  the  original 
publishers. 


X'RAY 


Sr  I    DIES 


GENERAL  ELECTRIC  COMPANY 
SCHENECTADY,  N.  Y. 


OCTOBER,  1919 


Y-1368 


INDEX 

Metallic  Tungsten  and  Some  of  Its  Applications  (1912)    .        .        .... 

W.    D.    COOLIDGE 

The  Effect  of  Space  Charge  and  Residual  Gases  on  Thermionic  Currents  in 

High  Vacuum  (1913)         .        .        .v      ...        .        .        ,        >        .        .11 

I.  LANGMUIR 

A  Powerful  Roentgen-ray  Tube  with  a  Pure  Electron  Discharge  (1913)         .        .     39 

W.  D.  COOLIDGE 

Physical  Investigation  Work  in  Progress  on  Tubes  and  Accessories  (1915)    .        .     55 

W.  D.  COOLIDGE 

A  New  Device  for  Rectifying  High  Tension  Alternating  Currents  (1915)       .        .     63 

SAUL  DUSHMAN 

A  Model  X-Ray  Dark-Room  (1915)      .        .        .        . 79 

W.  P.  DAVEY 

A  Summary  of  Physical  Investigation  Work  in  Progress  on  Tubes  and  Accessories 

(1915)          .        .        .        ......   \  ...        .        ...        .        .        .83 

W.  D.  COOLIDGE 

Roentgen  -rays  from  Sources  Other  Than  the  Focal  Spot  in  Tubes  of  the  Pure 

Electron  Discharge  Type  (1917) 95 

W.  D.  COOLIDGE  and  C.  N.  MOORE 

A  Portable  Roentgen-ray  Generating  Outfit  (1918)     .        •    *•        •        •..-•'      .107 

W.  D.  COOLIDGE  and  C.  N.  MOORE 

A  New  Radiator  Type  of  Hot  Cathode  Roentgen-ray  Tube  (1919)         .        .        .117 

W.  D.  COOLIDGE 

The  Use  and  Abuse  of  Roentgen-ray  Tubes  (1918) .123 

C.  N.  MOORE 

The  Radiator  Type  of  Tube  (1918) 127 

W.  D.  COOLIDGE 

Apparatus  for  Portable  Radiography  (1919) 133 

W.  D.  COOLIDGE 


Roentgen-ray  Spectra  (1915) 153 

A.  W.  HULL 

The  High  Frequency  Spectrum  of  Tungsten  (1916) 161 

A.  W.  HULL  and  MARION  RICE 

432353 


INDEX  — Continued 

PAGE 

The   Maximum  Frequency  of  X-Rays  at  Constant  Voltages  Betvveen  30,000 

and  100,000  (1916)    .        . 105 

A.  W.  HULL 

The  Law  of  Absorption  of  X-Rays  at  High  Frequencies  (1916)       .        .        .        .107 

A.  W.  HULL  and  MARION  RICE 

The  X-Ray  Spectrum  of  Tungsten  (1916)    ...        . 169 

A.  W.  HULL 

A  New  Method  of  X-Ray  Crystal  Analysis  (1917)      .        .        .        .        .  .    179 

A.  W.  HULL 

A  New  Method  of  Chemical  Analysis  (1919) 207 

A.  W.  HULL 

Positions  of  Atoms  in  Metals  (1919)      .        ,       ..        .        .        ...        .        .213 

A.  W.  HULL 


Some  Interesting  Applications  of  the  Coolidge  X-Ray  Tube  (1914)        .        .        .231 

W.  P.  DAVEY 

Radiography  of  Metals  (1915) 239 

W.  P.  DAVEY 

X-Ray  Examination  of  Built-up  Mica  (1915)       .        .  .        .        .        .        ..    247 

C.  N.  MOORE 

Application  of  the  Coolidge  Tube  to  Metallurgical  Research  (1915)        .        <        .   251 

W.  P.  DAVEY 

The  Effect  of  X-Rays  on  the  Length  of  Life  of  Tribolium  Confusum  (1917)         .255 

W.  P.  DAVEY 

Prolongation  of  Life  of  Tribolium  Confusum  Apparently  Due  to  Small  Doses  of 

X-Rays  (1919)   .        .     •  .        .        .        .        .        . 267 

W.  P.  DAVEY 


X-Rays  (1915)   ...  .    273 

W.  P.  DAVEY 


X'RAY  STUDIES 


METALLIC  TUNGSTEN  AND  SOME  OF  ITS  JAPPiJICAtlJG&S?*-V: 

BY  W.  D.  COOLIDGE 

Tungsten  does  not  occur  as  such  in  nature ;  but  in  the  form  of  compounds  it  is  pretty 
well  distributed.  The  most  important  ores  are  sheelite  or  calcium  tungstate  and  wolf- 
ramite or  iron-manganese  tungstate.  The  principal  source  of  the  ore  at  this  time  in 
this  country  is  Boulder  County,  Colorado. 

From  the  ore  it  is  a  simple  matter  to  get  the  yellow  oxide  (trioxide)  of  tungsten. 
And  the  trioxide  may  be  reduced  in  various  ways,  as  with  hydrogen,  zinc  and  carbon, 
to  metallic  tungsten. .  The  product  so  obtained  is  in  the  shape  of  a  powder  ranging  in 
color  from  gray  to  black  depending  upon  the  fineness  of  its  state  of  subdivision. 

Owing  to  its  very  high  melting  point,  it  was  for  a  long  time  impossible  to  get  the  pure 
powdered  metal  into  the  form  of  a  dense  coherent  homogeneous  mass.  Two  Austrians, 
Messrs.  Just  and  Hanaman1,  working  in  Vienna,  finally  succeeded,  however,  in  producing 
the  pure  metal  in  this  condition  and  in  filamentary  form,  and  by  using  it  as  an  incan- 
descent filament  became  the  inventors  of  the  tungsten  lamp. 

One  of  their  processes  consisted  in  first  mixing  the  finely  divided  tungsten  powder 
with  a  carbonaceous  binding  agent.  The  plastic  mass  so  obtained  was  then  extruded 
through  a  small  orifice.  The  resulting  thread  was  heated  in  the  absence  of  air,  to  car- 
bonize the  binder.  The  fragile  body  so  produced  consisted  of  tungsten  powder  and  car- 
bon and  was  electrically  conducting.  If  it  had  been  used  in  this  condition  it  would  have 
been  worthless  owing  to  the  presence  of  carbon  which  lowers  the  melting  point  and  be- 
sides this,  slowly  distills  out  in  a  vacuum,  causing  a  blackening  of  the  lamp  globe.  But 
Just  and  Hanaman  found  that  the  carbon  could  be  removed  by  heating  the  filament  to 
a  high  temperature,  by  passing  current  through  it,  in  an  atmosphere  of  hydrogen  and 
water  vapor.  This  high  temperature  treatment  not  only  removes  carbon  but  also 
causes  the  filament,  under  the  action  of  capillary  forces,  to  shrink  to  a  dense  homogen- 
eous body. 

Various  other  processes  were  subsequently  devised  for  making  small  filaments  of 
pure  tungsten.  But  these  were  all  alike  in  this,  that  they  started  with  the  finely 
divided  metal  or  oxide  or  some  other  compound  of  the  metal.  By  the  help  of  some 
agglomerant,  the  finely  divided  material  was  then  worked  into  a  plastic  mass.  This 
was  then  brought  into  filatnentary  form  by  extrusion  through  a  die,  and  was  then 
raised  to  a  high  temperature  under  such  conditions  that  nothing  but  pure  tungsten 
remained,  the  filament  sintering  under  the  application  of  the  high  temperature  to  a 
dense  coherent  body. 

Filaments  produced  in  this  way  showed  the  melting  point  of  pure  tungsten  (about 
3000  deg.  C.  )  And  they  were  very  satisfactory  as  lamp  filaments  except  in  one  re- 
spect— they  were  weak  mechanically.  They  were,  like  spun  glass,  highly  elastic  but 
absolutely  brittle ;  that  is,  they  could  not,  by  cold  blending,  be  given  the  slightest  per- 
manent deformation. 

In  the  above  mentioned  brittle  condition,  metallic  tungsten  had,  as  such,  but  one 
single  commercial  application,  namely  as  a  lamp  filament.  This  application  was  how- 
ever extremely  important — so  much  so  that  a  vast  amount  of  the  most  painstaking  in- 
vestigation work  was  carried  out  both  in  this  country  and  abroad  in  the  endeavor  to 
produce  a  ductile  form  of  tungsten.  Without  some  such  enormous  commercial  interest 
at  stake,  it  seems  extremely  doubtful  whether  the  product  sought  for  would  ever  have 

*  Copyright,  1912,  by  American  Institute  of  Electrical  Engineers. 
1.     U.  S.  Patent  Xo.  1,018,502. 


been  produced.  "For  no  one  knew  but  what  metallic  tungsten,  like  the  silicon  of  today, 
was,  at  ^rpLOi&^tsitopctattir^V inherently  brittle.  And  the  evidence  in  support  of  the 
theory  "that"  it  was  inherently  brittle  was  almost  overwhelming. 

At  a  meeting  of  the  Institute  in  May,  19102,  the  author  presented  a  paper  telling  of 
the  successful  reaching  of  the  goal — the  production  of  ductile  tungsten.  At  that  time 
samples  of  drawn  wire  were  shown  and  the  hope  was  expressed  that  within  a  short 
time  laboratory  processes  could  be  brought  to  a  point  where  the  public  could  profit  by 
the  new  invention.  This  development  has  taken  place  so  rapidly  that  today  the  bulk  of 
the  world's  supply  of  tungsten  lamps  is  made  from  drawn  wire. 

As  has  been  said  above,  so  long  as  tungsten  was  a  brittle  metal  it  found  but  one 
single  technical  application.  But  with  the  advent  of  ductile  tungsten,  the  whole  situa- 
tion is  changed,  and  the  metal  has  now,  apart  from  its  use  as  a  lamp  filament,  assumed  a 
very  considerable  degree  of  technical  importance. 

Before  proceeding  to  a  discussion  of  some  of  the  new  applications  it  may  be  well  to 
summarize  here  some  of  the  most  important  properties  of  wrought  tunsten.  Most  of 
these  data  have  already  been  published.3 

Physical  Properties.  Wrought  tungsten  is  a  bright  steel-colored  metal  having  the 
same  density  as  gold,  19.3.  (This  varies  somewhat  with  the  amount  of  mechanical 
working  which  the  specimen  has  had.) 

The  strength  and  pliability  both  increase  with  the  amount  of  mechanical  working. 
The  fracture  may  be  very  coarsely  crystalline,  or  it  may  resemble  that  of  a  very  fine 
grained  tool  steel  or  it  may  be  fibrous  and  silky  in  appearance,  or  it  may  lie  anywhere 
between  these  extremes,  the  appearance  in  each  case  depending  upon  the  chemical 
purity  and  upon  the  preceding  thermal  and  mechanical  treatment.  The  tensile 
strengths  measured  have  ranged  from  460,000  Ib.  (208,652  kg.)  per  sq.  in.  (6.45  sq.  cm.) 
for  a  wire  0.005  in.  (0.127  mm.)  in  diameter  to  610,000  Ib.  (276,691  kg.)  for  a  0.0012-in. 
(0.0305  mm  )  wire. 

Tungsten  is  hardened  by  working  but  not  by  heating  and  quenching.  Similarly 
tungsten  containing  carbon  is  not  appreciably  affected  by  quenching.  The  hardness 
imparted  by  working  may  be  entirely  removed  by  carrying  the  metal  to  white  heat. 

The  ductility  is  extreme,  as  is  shown  by  the  fact  that  wire  only  0.0006  in.  (0.0152 
mm.)  in  diameter  is  now  produced  in  large  quantity. 

Tungsten  is  non-magnetic. 

The  electrical  resistivity  at  25  deg.  C.,  expressed  in  microhms  per  centimeter  cube 
is  6.2  for  the  hard  drawn  wire  and  5.0  for  the  same  annealed.  The  corresponding  data 
for  annealed  copper  and  annealed  platinum  are  1.87  and  11.1  respectively. 

The  temperature  coefficient  of  electrical  resistivity  per  degree  between  0  deg.  and 
170  deg.  C.  is  0.0051. 

Assuming  the  Franz-Wiedemann  law  to  hold  for  the  relation  of  heat  to  electrical 
conductivity,  we  may  calculate  the  heat  conductivity  of  annealed  wrought  tungsten  to 
be  0.37  times  that  of  copper  and  2.2  times  that  of  platinum. 

The  coefficient  of  heat  expansion  per  degree  from  20  deg.  to  100  deg.  C.,  is  336  X 
10~8,  which  is  about  0.26  that  of  platinum. 

Chemical  Properties.  Wrought  tungsten  does  not  tarnish  upon  standing  in  the  air. 
Upon  heating  it  to  a  temperature  of  three  or  four  hundred  degrees,  however,  it  oxidizes 
superficially  and  turns  blue  just  as  steel  does.  At  bright  red  heat  the  oxide  volatilizes 
and  the  metal  wastes  away  more  or  less  rapidly,  depending  upon  the  temperature. 

2.  Transactions  A.I.E.E.  Vol.  29,  Part  II,  pp.  961  to  965. 

3.  C.  G.  Fink,  Transactions  Am.  Electrochem.  Soc.,  Vol.  17,  pp.  229  to  234  (1910). 
W.  E.  Ruder,  Jour.  Amer.  Chem.  Soc.,  April,  1912. 

6 


It  is  quite  resistant  to  the  action  of  most  acids,  being  entirely  unaffected  at  room 
temperature  by  either  dilute,  or  concentrated  hydrofluoric,  hydrochloric,  nitric,  and  sul- 
phuric acids.  With  aqua  regia  at  room  temperature  the  action  is  very  slight.  At  a 
higher  temperature,  110  deg.  C.,  there  is  no  action  in  the  case  of  hydrofluoric,  con- 
centrated nitric  and  dilute  sulphuric  acids,  while  the  action  is  but  slight  in  the  case  of 
dilute  and  concentrated  hydrochloric,  concentrated  sulphuric,  dilute  nitric,  and  aqua 
regia.  There  is  no  action  in  the  case  of  a  mixture  of  sulphuric  and  chromic  acids,  but 
the  metal  dissolves  rapidly  in  a  mixture  of  hydrofluoric  and  nitric  acids. 

An  aqueous  solution  of  caustic  potash  has  no  effect  on  wrought  tungsten,  but  the 
fused  salt  does  attack  the  metal  slowly. 

In  aqueous  solutions  of  sodium  or  potassium  carbonate  or  mixtures  of  the  two, 
tungsten  dissolves  slowly,  the  action  being  considerably  hastened  by  the  addition  of 
potassium  nitrate. 

ELECTRICAL  CONTACTS  OF  WROUGHT  TUNGSTEN 

Under  the  conditions  pertaining  in  many  electrical  make  and  break  devices,  as  in 
magnetos,  spark  coils,  voltage  regulators,  railway  signal  relays,  telegraph  and  telephone 
relays,  telegraph  sending  keys,  etc.,  wrought  tungsten  has  proved  to  be  far  superior  to 
platinum  or  platinum-iridium  for  the  contact  points. 

This  was  not  in  any  sense  an  obvious  application,  for  tungsten  is  not,  like  platinum, 
a  difficultly  oxidizable  metal.  It  might  well  have  been  assumed  that  under  the  heat  of 
the  minute  arcs  which  form  when  the  contacts  are  separated,  the  tungsten  would  oxidize 
at  the  points  where  arcing  has  taken  place,  and  that  non-conducting  layers  would  thus 
be  formed  which  would  produce  a  high  and  variable  contact  resistance.  In  fact,  our 
first  experiments  bore  out  this  theory.  But  subsequent  work  showed  that  the  difficulty 
in  these  early  experiments  came  from  the  fact  that,  at  the  time,  we  were  unable  to  make 
a  good  thermal  and  electrical  joint  between  the  tungsten  and  the  contact  carrying  mem- 
bers. With  the  attainment  of  a  good  conducting  joint,  our  results  changed  completely. 
The  contacts  no  longer  rose  to  the  same  high  temperature  and  the  oxidation  decreased 
to  little  or  nothing.  Moreover,  in  case  there  was  any  oxidation,  it  was  to  the  lower  and 
electrically  conducting  oxides. 

Tungsten  contacts  wear  longer  than  those  of  platinum  or  platinum-iridium.  This  is 
doubtless  due  largely  to  the  lower  vapor  pressure.  At  temperatures  at  which  platinum 
volatilizes  badly,  tungsten  has  a  very  low  vapor  pressure.  Besides  this  the  heat  con- 
ductivity of  tungsten  is  more  than  twice  that  of  platinum  and  as  a  result,  the  contact 
faces  do  not  rise  to  the  same  high  temperature.  (In  comparison  with  platinum-iridium 
alloys,  the  ratio  of  heat  conductivities  is  still  more  favorable  to  tungsten.)  In  con- 
nection with  the  life  of  contacts,  another  important  consideration  is  that  of  hardness. 
Tungsten  is  so  hard  that  it  does  not  batter  down  at  all  under  the  continual  hammering 
which  the  contacts  get  in  service. 

Tungsten  contacts  show  less  tendency  to  stick  than  do  contacts  of  platinum  or 
platinum-iridium.  This  is  to  be  attributed  in  part  to  the  higher  melting  point  of 
tungsten.  There  seems  to  be  another  factor  here,  however,  for  while  we  are  able,  by 
proper  manipulation,  to  securely  weld  together  two  pieces  of  platinum  at  a  temperature 
considerably  below  the  melting  point,  it  has  not  as  yet  been  possible,  except  by  actual 
fusion,  to  produce  anything  more  than  a  very  weak  adhesion  between  two  pieces  of 
tungsten. 

One  minor  and  unexpected  advantage  connected  with  the  use  of  tungsten  contacts 
consists  in  the  fact  that  they  seem  less  sensitive  to  the  accidental  presence  of  oil  than  do 
platinum  contacts. 


Allusion  has  already  been  made  to  the  difficulty  at  first  experienced  in  producing 
satisfactory  thermal  contact  between  tungsten  and  the  metal  comprising  the  contact 
carrying  member.  This  was  due  to  the  fact  that  tungsten  cannot  be  satisfactorily 
soldered  by  any  of  the  ordinary  processes.  This  difficulty  has  been  overcome  in  the 
following  way:  The  little  disc  of  tungsten,  which  is  to  serve  as  contact  point,  is  at- 
tached by  means  of  copper  to  the  head  of  an  iron  tack.  Copper  does  not  alloy  with 
tungsten,  but  under  suitable  conditions,  it  wets  it,  and  then  adheres  firmly.  This  gives 
a  joint  of  high  thermal  and  electrical  conductivity  between  the  tungsten  and  the  head 
of  the  iron  tack.  The  shank  of  the  tack  is  then  either  pressed  in,  or  brazed,  or  riveted  to 
the  contact  carrying  member. 

WROUGHT  TUNGSTEN  AS  TARGET  IN  A  ROENTGEN  (X-RAY)  TUBE 

This  has  proved  to  be,  both  from  the  scientific  and  practical  points  of  view,  an  ex- 
ceptionally interesting  application. 

Until  recently  platinum  has  been  almost  universally  regarded  as  the  best  target 
material,  and  it  has  so  long  held  undisputed  sway  in  this  field  that  the  Roentgen  ray 
worker  has  come  to  look  upon  its  limitations  as  inherent  in  the  Roentgen  tube. 

With  the  advent  of  dense,  forged  pieces  of  pure  malleable  tungsten  the  possibilities 
of  the  Roentgen  tube  are  greatly  extended. 

The  desiderata  in  a  material  to  be  used  as  the  anticathode  or  target  are  the  fol- 
lowing: 

1.  High  specific  gravity. 

2.  High  melting  point. 

3.  High  heat  conductivity. 

4.  Low  vapor  pressure  at  high  temperature. 

The  reasons  why  the  above  qualities  are  desirable  follow  readily  from  a  brief  con- 
sideration of  the  theory  of  Roentgen-ray  production. 

From  the  concave  cathode,  electrically  charged  particles,  the  electrons,  are  shot  out 
at  high  velocity  in  a  direction  normal  to  the  surface.  The  paths  of  these  particles  con- 
verge and  the  target  is  placed  at  or  near  the  point  of  strongest  convergence,  the  focus 
point.  When  the  electron  meets  an  obstruction,  as  the  target,  its  velocity  is  reduced, 
and  the  denser  the  target  the  more  rapid  is  the  deceleration.  The  more  rapid  the  de- 
celeration the  greater  is  the  amplitude  of  the  electromagnetic  pulse,  the  Roentgen-ray 
sent  out.  Here  then  is  a  need  for  high  specific  gravity;  that  of  forged  tungsten  is  but 
little  less  than  that  of  platinum. 

In  modern  Roentgen-ray  pratice,  powerful  apparatus,  running  sometimes  to  a 
capacity  of  10  and  even  15  kilowatts,  is  used  to  excite  the  tube.  The  greater  part  of  the 
energy  delivered  to  the  tube  is  transformed  into  heat  at  the  point  where  the  cathode 
rays  bombard  the  target.  Where  platinum  is  used  it  has  been  found  necessary,  to  pre- 
vent melting,  to  place  the  target  beyond  the  focus  of  the  cathode  so  as  to  spread  the 
bombardment  over  a  larger  area.  As  a  radiograph  is  a  shadow  picture,  and  as  the 
source  of  the  Roentgen-ray  is  the  bombarded  area  of  the  target,  this  enlarging  of  that 
area  is  clearly  an  undesirable  thing  to  do,  as  the  larger  area  will  mean  more  overlapping 
and  less  definition  in  the  resulting  picture.  In  this  way,  the  melting  point  of 
platinum  has  been  the  limiting  feature  of  the  Roentgen  tube.  The  capacity  of  the 
tube  has  been  increased  by  water  cooling  the  platinum  or  by  using  as  a  target  a  large 
mass  of  copper  having  a  very  thin  platinum  face.  But  the  limit,  although  raised  by 
these  artifices,  has  still  been  the  melting  point  of  the  platinum. 

Tungsten  has  a  much  higher  melting  point  (3000  deg.  C.  as  against  1755  deg. 
for  platinum),  and  so,  even  with  sharp  focusing  of  the  cathode  rays  on  the  target, 


permits  the  use  of  more  energy  than  has  hitherto  been  possible;  for  the  high  tempera- 
ture to  which  it  can  run  enables  it  to  radiate  more  heat,  and  its  better  heat  conduc- 
tivity permits  a  more  rapid  flow  of  heat  from  the  focus  spot  to  the  surrounding 
metal. 

Stability  of  vacuum  in  a  Roentgen  tube  is  of  the  utmost  importance,  as  the  character 
of  the  rays  is  so  largely  determined  by  the  vacuum.  Metal,  which  under  the  influence 
of  the  high  temperature  vaporizes  from  the  target,  condenses  on  the  glass  in  finely 
divided  form  and  absorbs  relatively  large  amounts  of  gas,  thus  changing  the  vacuum. 
At  high  temperatures  tungsten  vaporizes  least  of  all  the  metals. 

Two  distinct  types  of  tungsten  target  are  being  tried  out  experimentally  in  competi- 
tion with  one  another. 

The  first  of  these  consists  of  a  heavy  copper  block  with  a  disc  of  wrought  tungsten 
attached  to  the  face.  This  is  similar  to  the  platinum  targets  which  have  been  in  use  for 
some  years.  The  function  of  the  copper  is  simply  to  conduct  heat  away  from  the  tung- 
sten and  to  act  as  a  heat  storage  reservoir.  In  this  latter  capacity  it  is  much  more 
effective  than  would  at  first  seem  possible,  owing  to  the  fact  that  while  the  rate  of 
energy  input  is  high  the  time  is  correspondingly  short,  a  single  excitation  of  the  tube 
lasting  for  perhaps  only  a  half  second.  It  is  interesting  to  note  that  it  would  take  an 
energy  input  of  over  60  kilowatts  acting  for  a  second  to  raise  the  temperature  of  a  half 
pound  mass  of  copper  by  700  deg.  C. 

The  second  type  is  for  the  first  time  made  possible  by  the  advent  of  wrought  tung- 
sten. In  this  type  the  target  consists  entirely  of  refractory  metal,  and  this  is  allowed 
to  heat  up  to  such  temperatures  that  it  is  capable  of  radiating  relatively  large  quantities 
of  energy.  Tungsten  at  its  melting  point,  is  capable  of  radiating  about  375  watts  per 
sq.  cm.  This  means  that  a  tungsten  disc  3  cm.  in  diameter  and  0.2  cm.  thick  would,  if  its 
entire  mass  were  at  the  melting  point,  be  radiating  energy  at  the  rate  of  over  5  kilo- 
watts. (The  same  cylinder,  if  made  of  platinum  and  run  at  1750  deg.,  the  melting 
point  of  platinum,  would  radiate  only  about  one  twentieth  as  much.)  It  would  not  of 
course  be  practicable  to  run  the  entire  tungsten  mass  at  the  melting  point,  and,  owing 
to  the  resistance  to  heat  flow,  the  focal  spot  would  of  course  come  to  the  melting 
point  while  the  periphery  was  at  a  considerably  lower  temperature.  But  such  a  target 
would  at  least  be  able  to  radiate  continuously  relatively  large  amounts  of  energy 

The  target  has,  for  a  long  time,  been  the  most  vulnerable  point  in  the  tube,  but  this 
is  no  longer  true.  For  either  of  the  above  types  of  tungsten  target  can  be  made  so  good 
that  failure  of  the  tubes  as  they  are  now  built  will  be  due  to  other  causes. 

THE  TUNGSTEN  OR  MOLYBDENUM-WOUND  ELECTRIC  FURNACE 
This  has  been  fully  described  in  an  article  by  Winne  and  Dantsizen4.  This  is  not 
only  a  useful  adjunct  to  scientific  investigation  work;  but  has  already  become  a  very 
important  factory  tool.  When  wound  upon  a  body  of  alundum,  either  of  these  metals 
makes  possible  the  attainment  of  higher  temperatures  than  can  be  obtained  with  a 
platinum-wound  furnace.  And  as  higher  and  higher  melting  refractories  are  produced 
the  temperature  range  of  this  furnace  will  be  extended. 

TUNGSTEN  PROJECTILES 

The  use  of  wrought  tungsten  as  a  projectile  is  being  carefully  investigated.  It  offers, 
in  this  field,  possibilities  not  possessed  by  any  other  metal. 

The  present  small  arm  service  projectile  is  made  of  lead  with  a  jacket  of  copper- 
nickel  alloy.  The  principal  advantage  of  lead  over  iron,  which  would  ot  course  be 

4.     R.  Winne  &  C.  Dantsizen,  Transactions  Am.  Electrochem.     Soc.  Vol.  20,  pp.  287  to  290  (1911). 


cheaper,  is  that  it  has  a  higher  specific  gravity.  Because  of  this  fact  a  lead  bullet  will 
have  a  smaller  cross-section,  and  will  therefore  encounter  less  air  resistance  to  its  flight, 
than  will  an  iron  bullet  of  the  same  weight,  and  will  therefore  give  a  flatter  trajectory 
and  longer  range.  An  iron  bullet  of  the  same  diameter  as  the  lead  bullet  could  of  course 
be  given  the  same  weight  by  increasing  its  length.  But  this  would  at  once  necessitate 
giving  it  a  higher  rotational  velocity  to  keep  its  axis  tangential  to  its  flight.  To  impart 
this  added  rotational  velocity  calls  for  the  expenditure  of  energy  and  so  leaves  less  for 
velocity  of  translation.  The  lead  bullet  then  with  its  higher  density  makes  possible  a 
flatter  trajectory  and  longer  range.  With  the  exception  of  tungsten,  lead  is  the  densest 
metal  which  can  be  considered  for  this  purpose,  for  gold  is  the  cheapest  of  the  other 
elements  having  a  higher  specific  gravity  than  lead.  The  density  of  wrought  tungsten 
is  19.3  while  that  of  lead  is  11.5. 

For  military  purposes,  the  softness  of  lead  is  not  an  advantage,  a  soft  nosed  bullet 
being  tabooed  in  civilized  warfare.  For  this  reason  and  because  of  the  fact  that  it  is  too 
weak  to  hold  the  rifling,  it  has  to  be  jacketed  with  the  copper-nickel  alloy.  To  take  the 
rifling  and  to  act  as  a  gas  check,  the  tungsten  bullet  will  require  a  copper  band  or  its 
equivalent  at  the  base. 

The  hardness  and  high  tensile  .strength  of  wrought  tungsten  will  give  high  penetrat- 
ing power. 

The  high  melting  point  of  tungsten  will  prevent  it  from  being  harmfully  upset  at  the 
base  by  the  combined  action  of  the  high  temperature  and  rapid  impact  due  to  the  com- 
bustion of  the  powder  charge.  (An  unsymmetrical  upsetting  of  the  ba§e  of  a  projectile 
is  very  prejudicial  to  accuracy.) 

It  would  be  a  very  simple  matter  to  calculate  the  constants  of  the  trajectory  of  a 
tungsten  projectile  were  it  not  for  the  fact  that  the  high  density  removes  it  too  far  from 
the  present  base  line.  For  such  calculations  one  quantity  is  lacking,  the  so-called 
"form  factor."  This  factor  could  itself  be  calculated  if  it  contained,  as  the  name  would 
seem  to  imply,  only  the  dimensions  and  the  specific  gravity,  but  there  are  also  involved 
in  it  all  of  the  errors  due  to  simplifying  assumptions  which  have  been  made  in  con- 
nection with  the  mathematical  derivation  of  formulae.  It  therefore  becomes  necessary 
to  experimentally  determine  the  constants  of  the  trajectory  of  tungsten  bullets,  and 
this  work  is  now  being  carried  out. 

CHEMICAL  WARE  OF  TUNGSTEN 

Because  of  its  inertness  to  many  chemical  agents  wrought  tungsten  in  the  form  of 
dishes  is  certain  to  find  many  industrial  applications.  Small  dishes  have  already  been 
made  successfully,  and  work  is  already  under  way  looking  to  the  production  of  larger 
sizes. 


10 


[Reprinted  from  the  PHYSICAL  REVIEW,  N.  S.,  Vol.  II,  No.  6,  December,  1913.] 

THE   EFFECT  OF  SPACE   CHARGE  AND   RESIDUAL  GASES  ON 
THERMIONIC  CURRENTS  IN  HIGH  VACUUM  * 

BY  IRVING  LANGMUIR 
x  • 

When  a  carbon  or  metal  filament  is  heated  in  a  vacuum  and  surrounded  by  a  posi- 
tively charged  metal  cylinder,  it  is  well  known  that  electrons  are  given  off  by  the  hot 
solid.  This  effect  in  lamps  has  been  commonly  known  as  the  Edison  effect  and  has  been 
rather  fully  described  in  the  case  of  carbon  lamps  by  Fleming.1 

Richardson  and  others  have  studied  quantitatively  the  ionization  produced  by  hot 
solids,  especially  from  heated  platinum,  and  have  collected  a  large  amount  of  data.  It 
has  generally  been  found  that  the  saturation  current  is  independent  of  the  pressure  of 
the  gas  and  increases  rapidly  with  increasing  temperature  of  the  filament.  However, 
certain  gases  were  found  to  have  very  marked  effects ;  for  example,  traces  of  hydrogen 
were  found  to  increase  enormously  the  saturation  current  obtained  from  hot  platinum  .'- 
Recent  investigations  have  shown3  that  at  least  in  some  cases  the  current  is  due  to 
secondary  chemical  effects. 

Pring  and  Parker4  showed  that  the  current  obtained  from  incandescent  carbon 
could  be  cut  down  to  very  small  values  by  progressive  purification  of  the  carbon  and 
improvement  of  the  vacuum.  They  conclude  that  "the  large  currents  hitherto  ob- 
tained with  heated  carbon  cannot  be  ascribed  to  the  emission  of  electrons  from  carbon 
itself,  but  that  they  are  probably  due  to  some  reaction  at  high  temperatures  between 
the  carbon,  or  contained  impurities,  and  the  surrounding  gases,  which  involves  the 
emission  of  electrons."  Pring  and  Parker  observed  also  that  the  ionization  (or  rather 
thermionic  current)  "increased  only  very  slightly  with  the  temperature  above  1800 
degrees." 

The  effect  of  these  publications,  together  with  that  of  Soddy,5  who  noticed  similar 
effects  with  a  Wehnelt  cathode,  has  been  to  cast  doubt  on  the  existence  of  a  thermionic 
current  in  a  perfect  vacuum  and  from  pure  metals.  The  opinion  seems  to  be  gaining 
ground,  especially  in  Germany,  that  the  emission  of  electrons  from  incandescent  solids 
is  a  secondary  effect  produced  by  chemical  reactions,  or  at  least  is  caused  by  the  pres- 
ence of  gas. 

With  the  above-mentioned  exceptions,  it  has  generally  been  found  that  the  therm- 
ionic current  increased  with  the  temperature  at  a  very  high  rate.  The  relation  be- 


*  Copyright,  19)3,  by  American  Physical  Society, 
i  Phil.  Mag.,  42.  P.  52  (1896). 

s  H.  A.  Wilson,  Phil.  Trans.,  SOS,  243  (1903). 

3  Fredenhagen,  Ver.  d.  phys.  Ges.,  14,  384  (1912). 

*  Phil.  Mag.  23,  192  (1912). 
5  Nature,  77,  53  (1907). 


11 


tween  current  and  temperature  was  usually  accurately  represented  by  Richardson's 
equation 

_b_ 

(1)  *  =  aV^fe  T' 

where  a  and  b  are  constants  and  i  is  the  saturation  current  at  the  absolute  temperature 

r. 

If  the  older  values  of  a  and  b  as  found,  for  example,  for  carbon,  are  substituted  in 
the  above  equation  and  the  currents  for  very  high  temperatures  (above  2500  deg.)  are 
calculated,  values  of  many  amperes  or  even  thousands  of  amperes  per  square  cm.  are 
usually  obtained.  This  raises  the  question  why  in  ordinary  incandescent  lamps  very 
large  thermionic  currents  do  not  occur. 

There  is  every  reason  to  think  that  the  thermionic  current  from  tungsten  should  be 
fairly  large.  When  we  run  a  tungsten  lamp  up  to  2  or  even  2.5  times  its  normal  voltage 
(filament  temperature  2900-3400  deg.  K.)  we  should,  therefore,  expect  to  get  thermionic 
currents  of  several  amperes  between  the  two  ends  of  the  filament.  Simple  observation 
of  a  lamp  run  under  such  conditions  indicates  that  this  is  not  the  case.  For  example, 
consider  a  lamp  which  takes  110  volts  and  0.3  ampere  when  running  at  normal  specific 
consumption  (1.25  watts  per  candle).  By  raising  the  voltage  to  250,  the  temperature 
of  the  filament  will  be  brought  to  about  3000  deg.  K.  and  the  current  is  then  about  0.45 
ampere.  The  resistance  of  the  filament  has  thus  increased  from  366  ohms  up  to  555. 
The  total  surface  of  the  filament  is  nearly  half  a  square  cm.,  yet  it  is  evident  that  if 
there  is  any  thermionic  current  between  the  two  ends  of  the  filament,  it  cannot  exceed 
a  few  hundredths  of  an  ampere.  This  apparent  discrepancy  between  the  results  of 
calculation  by  Richardson's  equation  and  the  facts  observed  with  a  tungsten  lamp 
seemed  at  first  to  confirm  the  growing  opinion  that  in  a  very  high  vacuum  the 
thermionic  current  is  very  small,  if  not  entirely  absent. 

Experiments  on  the  Edison  effect  in  tungsten  lamps,  made  some  time  ago  by  the 
writer,  throw  a  great  deal  of  light  on  the  cause  of  the  apparent  failure  of  Richardson's 
equation  at  high  temperatures.  The  observations,  therefore,  seem  of  sufficient  interest 
to  warrant  their  publication. 

EXPERIMENTS  ON  EDISON  EFFECT  IN  TUNGSTEN  LAMPS 

Some  lamps  were  made  containing  two  single  loop  (hairpin)  tungsten  filaments  with 
separate  leading-in  wires.  Each  loop  could  thus  be  run  separately.  The  lamps  were 
given  a  specially  good  lamp  exhaust,  which  involved  heating  them  to  360  deg.  for  an 
hour  while  being  exhausted  with  a  mercury  pump.  A  trap  immersed  in  liquid  air  was 
placed  between  the  pump  and  the  lamp  to  condense  out  water  vapor,  carbon  dioxide 
and  mercury  vapor.  The  filaments  were  then  connected  in  series  and  the  lamps  run  at 
a  specific  consumption  of  about  1  watt  per  candle  for  fifteen  minutes,  to  drive  the  gas 
from  the  filaments.  The  lamps  were  then  sealed  off  from  the  pump  and  the  filaments 
were  again  heated,  this  time  being  run  at  0.4  watt  per  candle  for  a  few  minutes,  to  age 
the  filaments  and  improve  the  vacuum  (clean-up  effects). 

Experiments  were  then  undertaken  to  measure  the  thermionic  currents  that  flowed 
across  the  space  between  the  two  filaments  when  one  was  heated  to  various  tempera- 
tures while  the  other  was  connected  to  a  constant  source  of  positive  potential  of  about 
125  volts.  A  milliammeter  was  connected  in  series  with  the  cold  filament. 

When  the  temperature  of  the  cathode  filament  wras  raised  to  about  2000  deg.  K.  a 
current  of  about  0.0001  ampere  was  observed  to  flow  between  the  two  filaments.  As 
this  temperature  was  raised  the  thermionic  current  rose  very  rapidly,  until  at  about 
2200  deg.  K.  it  was  about  0.0006  ampere.  As  the  temperature  was  raised  above  2200 

12 


deg.  K.,  no  further  increase  in  the  thermionic  current  occurred,  even  when  the  filament  was 
heated  nearly  to  the  melting-point  (3540  deg.  K.)  -  By  raising  the  voltage  on  the  anode 
to  about  250  volts,  the  thermionic  current  increased  to  about  0.0015  ampere.  It  re- 
quired, however,  a  temperature  about  200  deg.  higher  to  reach  this  current  than  had 
been  found  necessary  to  reach  the  maximum  current  at  the  lower  voltage.  At  tem- 
peratures below  2200  deg.  K.,  the  current  was  practically  the  same  with  125  as  with  250 
volts. 

The  results  of  a  later  and  more  accurate  experiment  are  given  in  Fig  1.  The  fila- 
ment used  for  these  measurements  consisted  of  a  single  loop  of  drawn  tungsten  wire, 
of  diameter  0.0069  cm.  and  total  length  of  10.84  cm.  and  area  of  0.234  sq.  cm.  A 
similar  filament,  at  a  distance  of  about  1.2  cm.,  served  as  anode.  Both  filaments 
had  been  aged  a  couple  of  hours  at  high  temperature  and  the  vacuum  thus  obtained 
was  certainly  better  than  10~6  mm.1 

The  temperatures  of  the  filament  were  determined  from  the  relation 

11,230 

~  7.029  -logic//' 

where  H  is  the  intrinsic  brilliancy  of  the  filament  in  international  candles  per  sq.  cm. 
of  projected  area.2 

The  points  indicated  by  small  circles  (Fig.  1)  are  experimentally  determined.  Two 
different  anode  voltages  were  used:  240  and  120  volts,  with  respect  to  the  negative 
terminal  of  the  filament  which  served  as  hot  cathode.  The  voltage  used  to  heat  the 
cathode  varied  from  about  7  to  15  volts,  so  the  average  potential  difference  between 
anode  and  cathode  was  somewhat  less  than  240  and  120  volts.  The  curve  given  for  60 
volts  was  determined  from  other  experiments  devised  especially  to  determine  the  effect 
of  voltage  variations. 

It  is  seen  from  these  curves  that  at  low  temperatures  the  current  for  all  three 
voltages  is  the  same,  but  that  as  the  temperature  is  raised  the  currents  at  the  lower 
voltages  fall  below  those  for  higher  voltages  and  finally  each  in  turn  reaches  a  constant 
value. 

By  plotting  (log  i  —  %  log  7)  against  1/7  it  was  found  that  all  the  points  on  the 
240- volt  curve  up  to  a  temperature  of  about  2150  deg.  lay  very  close  to  a  straight 
line.  This  indicates  that  these  results  can  be  expressed  by  Richardson's  equation. 
From  the  slope  and  position  of  the  line,  the  values  of  a  and  b  of  Richardson's  equa- 
tion were  found  to  be 

a  =  27X!06  (amperes  per  sq.  cm.), 
b  =  55,600    (degrees) . 

The  heavy  black  curve  of  Fig.  1  was  calculated  by  plotting  Richardson's  equation, 
using  these  values  of  a  and  b.  The  agreement  between  this  curve  and  the  experiments 
at  low  temperature  is  nearly  perfect;  in  fact,  much  better  than  can  be  seen  from  Fig.  1 . 

It  is  seen  that  each  experimentally  determined  curve  can  be  divided  into  three 
parts : 

1 .  A  part  which  follows  Richardson's  equation  accurately. 

2.  A  part  which  consists  of  a  horizontal  straight  line;  that  is,  a  part  in  which  the 
current  is  independent  of  the  temperature  of  the  filament. 

3  A  transition  curve  between  these. 

The  horizontal  part  of  the  curve  was  of  particular  interest.  The  current  being  in- 
dependent of  the  temperature  of  the  filament  is  probably  independent  of  the  nature  of 

1  Judging  from  measurements  on  similar  lamps  made  by  means  of  the  "molecular"  gage  described  by  the  writer, 
PHYS.  REV.,  7(2).  337  (1913). 

•  The  derivation  of  this  formula  will  soon  be  published,  probably  in  the  PHYSICAL  REVIEW. 

13 


the  cathode.  It  seemed  possible,  however,  that  it  might  be  dependent  on  the  anode. 
Several  experiments  were  undertaken  to  determine  the  factors  which  governed  the 
value  of  this  new  kind  of  "temperature"  saturation  current.  It  was  found  that  it  was 
very  largely  affected  by  any  one  of  the  four  factors : 

1 .  Voltage  of  anode. 

2.  Presence  of  magnetic  field. 

3.  Area  of  anode. 

4.  Distance  from  anode  to  cathode. 

It  is  especially  noteworthy  that  none  of  these  factors  had  any  influence  on  the 
thermionic  current  over  the  first  part  of  the  curve;  i.e.,  that  part  which  follows 
Richardson's  equation.  That  is,  the  constants  a  and  b  were  not  affected  by  voltage,  or 
magnetic  field,  or  distance,  or  area  of  anode. 

After  trying  out  several  hypotheses 
which  suggested  themselves,  it  finally 
occurred  to  the  writer  that  this  tem- 
perature saturation  might  be  due  to  a 
space  charge  produced  by  the  electrons 
between  the  cathode  and  anode.  The 
theory  of  electronic  conduction  in  a 
space  devoid  of  all  positive  charges 
or  gas  molecules  seems  to  have  been 
strangely  neglected.  It  has  apparently 
always  been  taken  for  granted  that 
positive  ions  are  present,  or  at  least  a 
sufficient  amount  of  gas,  so  that  the 
motion  of  the  electrons  follows  the  laws 
of  diffusion.  J.  J.  Thomson1  gives  the 
differential  equations  that  apply  to 
the  calculation  of  electron  conduction 
through  space,  and  suggests  that  a 
method  for  the  determination  of  e/m 
could  be  worked  out  in  this  way.  He 
apparently  does  not  fully  integrate  the 
equations  or  realize  their  application 


2600'H 


to  ordinarv  thermionic  currents. 


THEORY  OF  ELECTRONIC  CONDUCTION  IN  A  SPACE  DEVOID  OF  MOLECULES  OR 

POSITIVE  IONS 

In  order  to  form  a  clear  conception  of  the  problem  before  us,  let  us  consider  (see 
Fig.  2)  two  infinite  parallel  planes,  A  and  B,  one  of  which,  A,  has  the  properties  of  an 
incandescent  solid;  that  is,  we  assume  that  it  emits  low  velocity  electrons  spontaneous- 
ly. The  other,  plane  B,  we  consider  to  be  positively  charged. 

Now  if  the  temperature  of  the  plate  A  is  so  low  that  few  or  no  electrons  are  emitted, 
then  the  potential  between  the  two  plates  will  vary  linearly  between  the  two,  as  indi- 
cated by  the  line  PT. 

As  the  temperature  of  A  is  raised,  electrons  are  emitted.  Under  the  influence  of  the 
field  these  pass  across  the  space  from  A  to  B  and  thus  constitute  a  current  of  magni- 
tude i  (per  sq.  cm.). 

1  Conduction  of  Electricity  through  Gases,  2d  edition,  p.  223. 


14 


These  electrons  move  with  a  velocity  which  depends  on  the  potential  drop  through 
which  they  have  passed.  Let  us  assume,  as  a  first  rough  approximation,  that  they  move 
with  constant  velocity  across  the  space.  Then  there  will  be  in  the  unit  volume  a  space 
charge  p  equal  to  i/v,  where  v  is  the  velocity  of  the  electrons.  If  the  velocities  are  uni- 
form, the  space  charge  will  be  uniform  and  it  follows  from  Poisson's  equation. 


(2) 

that 

(3) 


= -4- 4- =  — 4-JTO 

TT      2 


=  —  4?rp 


dx* 


Fig.  2. 


If  we  consider  p  constant  and  negative  (for  electrons),  we  see  from  this  equation 
that  the  potential  distribution  between  the  two  plates  takes  the  form  of  a  parabola,  as 
indicated  by  the  curve  PST. 

If  the  temperature  of  the  plate  A  be  increased 
still  further,  the  electron  current  increases  so  that  the 
potential  curve  finally  becomes  a  parabola  with  a 
horizontal  tangent  at  P. 

If  we  assume  that  the  electrons  are  given  off  from 
the  plate  A  with  practically  no  initial  velocity,  we  see 
that  the  current  cannot  increase  beyond  the  point 
where  the  potential  curve  becomes  horizontal  at  P, 
for  any  further  increase  of  current  would  make  the 
potential  curve  at  P  slope  downwards  and  the  electrons 
would  be  unable  to  move  against  this  unfavorable 
potential  gradient. 

In  other  words,  we  see  that  the  effect  of  the  space 
charge  is  to  limit  the  current.  A  further  increase  in 
the  temperature  of  the  plate  A  would  then  not  cause 
an  increase  of  current. 

Electron  Current  between  Parallel  Planes. — Let  us  now  attempt  a  more  rigorous 
solution  of  the  problem. 

It  has  been  shown  by  Richardson  and  others  that  the  mean  kinetic  energy  of  the 
thermions  is  closely  equal  to  that  of  gas  molecules  at  the  same  temperature.  This  in- 
dicates that  they  have  velocities  so  low  that  very  few  of  them  are  capable  of  moving 
against  a  negative  potential  of  more  than  a  couple  of  volts.  Since  the  voltages  applied 
to  the  anode  are  much  larger  than  this,  we  may  assume,  for  convenience,  that  the 
electrons  are  given  off  by  the  plate  A  without  initial  velocity. 

Now  let  V  be  the  potential  at  a  distance  x  from  the  plate  A .  The  kinetic  energy  of 
an  electron  when  it  has  traveled  the  distance  x  from  the  plate  will  thus  be 

The  current  (per  unit  area)  carried  by  the  electrons  at  any  place  will  be 
(5)  i  =  pv 

For  convenience,  we  take  e  and  p  positive  even  for  electrons.  Equation  (3)  thus 
becomes  (in  electrostatic  units). 

(6) 


15 


These  three  equations  enable  us  to  express  V  as  a  function  of  x  and  i.    By  eliminat- 
ing p  and  v  from  (4),  (5)  and  (6),  we  obtain 


(7)  =  2^    £    i    ; 

\e  V  V 

Multiply  this  by  2-dV/dx  and  integrate 

' 


Now  if  there  is  an  opposing  (negative)  potential  gradient  at  the  surface  of  the  plate 
A,  no  current  will  flow.  It  there  is  a  positive  potential  gradient  all  the  electrons  that 
are  given  off  from  the  plate  A  will  reach  B,  so  that  the  current  that  flows  will  be  de- 
termined by  Richardson's  equation.  The  case  that  we  are  interested  in  is  that  in  which 
the  current  is  less  than  the  saturation  current  and  is  determined  by  the  voltage  of  the 
anode.  Evidently  for  this  case  the  potential  gradient  at  the  plate  A  is  zero;  that  is, 


fdV_\  =Q 

\dx  Jo 


whence  from  (8)  by  extracting  the  square  root : 

dV 


Integrating  and  solving  for  i,  we  obtain : 


This  equation1  gives  the  maximum  electron  current  density  between  two  infinite 
parallel  plates  with  the  distance  x  between  them  and  with  a  potential  difference  V. 
This  equation  holds  only  where  the  initial  velocity  of  the  electrons  at  the  plate  A  is 
negligible  compared  to  that  produced  by  the  potential  V.  It  does  not -hold  at  such 
high  voltages  that  the  electrons  move  with  velocities  approaching  that  of  light. 

Taking  e/m  =  1.77 X 107  E.M.  units,  reducing  to  E.S.  units  and  substituting  in  (10) 
and  then  reducing  to  volt,  ampere  units,  we  obtain  from  equation  (10) : 

_6FI( 
xz 

where  i  is  the  maximum  current  density  in  amperes  per.sq.  cm.,  x  is  the  distance  be- 
tween the  plates  in  centimeters,  and  V  is  the  potential  difference  in  volts. 

Electron  Current  between  Concentric  Cylinders. — Let  us  consider  a  wire  of  radius  a 
placed  in  the  axis  of  a  cylinder  of  radius  r.  Let  i  be  the  thermionic  current  per  unit  of 
length  from  the  wire. 

For  the  case  of  symmetrical  cylindrical  coordinates,  Poisson's  equation  becomes2 

l_df  dV\ 
r  dr\  dr  J 
or 


1  Since  submitting  this  paper  for  publication  the  attention  of  the  writer  has  been  called  to  the  fact  that   C.  D.  Child 
(PHYS.  REV.,  S2,  492,  1911),  has  already  derived  this  equation.    He  has,  however,  applied  it  only  to  the  case  where  the  con- 
duction takes  place  solely  by  positive  ions. 

2  Weber's  Differential  Gleichungen,  1900,  Vol.  1,  p.  98. 

16 


The  equation  corresponding  to  (5)  is 

(13)  i  =  2irrpv. 

These  two  equations,  together  with  equation  (4),  which  also  applies  to  this  case,  give  us 

d2V    dV 

(14)  T~W+~dr'  = 

This  equation  probably  cannot  be  directly  integrated,  but  it  is  possible  to  obtain  a 
result  in  terms  of  a  series.     The  final  solution  takes  the  form 


(15) 


.     2V2 

T 


I  e    Va 
\  m  rB2' 


where  13  is  a  quantity  which  varies  from  0  to  1 .    The  value  of  /3  can  be  obtained  by  sub- 
stituting equation  (15)  in  (14)  and  placing 


(16) 

Equation  (14)  is  thus  reduced  to 

(17)       .  3|3—  - 


=  ae. 


The  solution  of  this  equation  gives 


(18) 
where 


Mr.  E.  Q.  Adams,  of  this  laboratory,  has  calculated  the  values  of  j3  for  various  values 
of  r/a  and  has  shown  that  the  value  of  /3  rapidly  approaches  unity  and  that  for  all 
values  of  r/a  greater  than  10,  /3  may  for  most  purposes  be  taken  equal  to  unity.  The 
following  table  was  prepared  from  Mr.  Adams'  data : 


TABLE  I 


r/a                                               (F 

r/a 

' 

1.00 

0.000 

5.0 

0.755 

1.25 

0.045 

6.0 

0.818 

1.50 

0.116 

7.0 

0.867 

1.75 

0.200 

8.0 

0.902 

2.00 

0.275 

9.0 

0.925 

2.50 

0.405 

10.0 

0.940 

3.00 

0.512 

15.0 

0.978 

4.00 

0.665 

CO 

1.000 

Since  in  practical  cases  the  diameter  of  the  cylinder  around  the  wire  is  usually  much 
more  than  ten  times  that  of  the  wire,  the  formula  (1)  may  usually  be  written 


2v/2    \eVl 


(19) 


That  is,  the  maximum  electron  current  from  a  small  wire  is  independent  of  the  dia- 
meter of  the  wire,  inversely  proportional  to  the  radius  of  the  enclosing  cylinder  and 
proportional  to  V*. 


Substituting  numerical  values  for  e/m  and  reducing  to  ordinary  electrical  units 
(volts,  amperes,  cm.)  equation  (19)  becomes 

yl 

(20)  *  =  14.65  X10-6  —  . 

r 

Electron  Current  between  Electrodes  of  Other  Shapes.  —  It  can  be  shown  that  between 
electrodes  of  any  shape  the  maximum  electron  current  varies  with  F?.  Let  us  consider 
a  system  in  which  we  have  the  maximum  electron  current  with  the  potential  difference 
V.  Then  Poisson's  equation 

(2)  Ar  =  4xp 

holds  for  such  a  space,  as  well  as  the  two  equations 

(5)  i  —  pv 

and 

(4)  ±mv2  =  Ve. 

Now  let  us  increase  the  voltage  in  the  ratio  1  :n  and  increase  the  current  in  the  ratio 
1  :n%.  Equations  (5)  and  (4)  thus  become 

»i*  =  pv 

=  nVe. 


Eliminating  v  from  these,  and  solving  for  p,  we  get 


From  this  we  see  that  p  has  been  increased  n  fold.  However,  since  V  has  been  in- 
creased also  n  fold,  Poisson's  equation  (2)  still  holds.  Hence  we  see  that  increasing  the 
current  by  a  factor  n?  and  increasing  the  voltage  by  the  factor  n,  leads  to  a  condition 
which  still  satisfies  the  three  equations  (2),  (4),  and  (5).  This  is,  however,  equivalent 
to  increasing  the  current  proportionally  to  V*.  We  thus  see  that  whatever  the  con- 
figuration of  the  electrodes,  the  maximum  electron  current  varies  with  V$. 

DISCUSSION  OF  THE  THEORY 

The  foregoing  theoretical  considerations  have  indicated  that  in  a  space  devoid  of 
positive  ions,  or  gas  molecules,  the  space  charge  caused  by  the  electrons  limits  the  cur- 
rent that  flows  between  a  hot  cathode  and  cold  anode  under  a  given  difference  of  poten- 
tial. It  now  remains  to  compare  the  maximum  currents  obtained  in  the  experiments, 
with  those  calculated  from  the  equations  that  have  been  derived. 

Let  us  consider  the  data  given  in  Fig.  1.  The  maximum  electron  current  at  120 
volts  was  0.000426  ampere  from  the  hot  filament,  or  0.00182  ampere  per  sq.  cm.  At 
240  volts  the  total  current  was  0.00130  ampere,  or  0.0055(5  ampere  per  sq.  cm.  Since 
we  are  not  dealing  with  parallel  plane  electrodes,  nor  with  concentric  cylinders, 
neither  equation  (11)  nor  (20)  will  apply  rigorously.  We  are,  however,  mainly  con- 
cerned in  determining  whether  the  space  charge  is  an  adequate  explanation  of  the 
observed  limitation  of  the  thermionic  current,  and  therefore  can  test  out  the  two 
equations  by  calculating  the  distances  which  would  have  to  exist  between  plane  or 
cylindrical  electrodes  in  order  to  give  the  observed  thermionic  currents. 

We  will  first  test  out  equation  (11),  which  should  apply  to  parallel  plane  electrodes. 
Let  us  take  the  observed  values  of  V  and  i  and  calculate  x.  We  thus  obtain: 

For  7=120  volts  and  *  =  0.  00182,  we  find  x=  1.30  cm.; 
V  =  240  volts          i  =  0.00556,  we  find  x  =  1  .25  cm  . 

4 

18 


Our  formula  thus  indicates  that  the  current  density  between  parallel  plane  elec- 
trodes about  1 .3  cm.  apart  would  be  the  same  as  that  observed.  This  is,  however,  very 
close  to  the  actual  distance  between  the  electrodes.  The  very  close  agreement  is  prob- 
ably due  to  the  counter-balancing  of  two  factors:  first,  the  weakening  of  the  elec- 
trostatic field,  due  to  the  flare  of  the  lines  of  force  around  the  wires,  and  second,  the  re- 
duction of  the  intensity  of  the  space  charge  owing  to  this  same  flare. 

Let  us  now  test  the  equation  (20),  which  should  apply  to  concentric  cylinders. 
Since  the  wire  was  10.8  cm.  long,  the  values  of  i  (amperes  per  cm.)  were  0.0000394  at 
120  volts  and  0.000120  at  240  volts.  Substituting  these  values  in  (20)  and  solving 
for  r  we  obtain 

at  120  volts  r  =  490.  cm., 
240  volts  r  =  450.  cm. 

We  see  that  the  radius  of  a  cylindrical  anode  would  have  to  be  very  large  (470  cm.), 
in  order  to  give  only  the  observed  thermionic  current.  This  result,  however,  appears 
perfectly  reasonable,  for  the  field  produced  by  a  small  wire  anode  is  naturally  much 
weaker  than  that  produced  by  a  cylindrical  anode  surrounding  the  cathode. 

The  numerical  values  obtained  from  these  equations  are  certainly  of  the  right  order 
of  magnitude  and  agree  as  well  with  the  experimental  results  as  could  be  expected  when 
the  shape  of  the  electrodes  departs  so  far  from  those  assumed  in  the  calculations. 

We  have  seen  that  the  theory  leads  us  to  the  conclusion  that  for  electrodes  of  any 
shape  the  maximum  thermionic  current  should  vary  with  F*.  This  can  be  readily  tested 
out  from  the  experiment.  The  ratio  of  the  currents  at  the  two  voltages  is  3.05.  Taking 
the  2/3  power  of  this  ratio,  the  voltage  ratio  should  be  2.10,  whereas  actually  it  was  2.0. 
However,  it  must  "be  remembered  that  the  voltage  of  the  anode  was  measured  from  the 
negative  end  of  the  filament,  so  that  the  average  voltage  drop  from  anode  to  cathode 
was  about  seven  or  eight  volts  less  than  those  given.  This  would  give  for  the  voltage 
ratio  232:112,  or  2.07,  as  against  the  2.10  calculated  by  the  three-halves  power  law. 

Several  experiments  have  been  undertaken  to  study  the  relation  of  maximum  cur- 
rent to  voltage  over  a  wide  range  of  voltage.  Voltages  from  10  volts  up  to  800  volts 
have  been  tried  and  the  results  plotted  on  logarithmic  paper.  From  the  slopes  of  the 
resulting  lines  the  exponent  of  the  voltage  was  calculated.  Values  varying  from  1 .5 
to  1.7  have  been  obtained.  Under  certain  conditions,  to  be  described  in  more  detail 
later,  values  much  higher  than  these  are  sometimes  obtained. 

The  above  considerations  indicate  that  the  space  charge  produced  by  the  electrons 
is  a  sufficient  cause  for  the  limitation  of  current  observed  in  the  experiments. 

EFFECT  OF  RESIDUAL  GASES  ON  CONSTANTS  OF  RICHARDSON'S  EQUATION 

In  some  of  the  early  experiments  on  the  thermionic  current  between  two  tungsten 
filaments,  extremely  variable  results  were  obtained  for  the  constants  of  Richardson's 
equation.  In  many  cases,  however,  when  anode  potentials  as  high  as  150  or  250  volts 
were  applied,  a  blue  glow  appeared  in  the  lamp,  indicating  ionization  of  the  residual  gas. 
To  get  rid  of  this,  the  two  filaments  were  connected  in  series  and  both  heated  to  a  very 
high  temperature  (2900  deg.  K.)  for  a  few  minutes.  This  "cleans  up"  the  vacuum 
to  an  extremely  high  degree.  The  writer  is  publishing  a  series  of  articles  in  the 
Journal  of  the  American  Chemical  Society  on  the  clean  up  of  various  gases  in  a  tungsten 
lamp.  It  has  been  found  that  with  a  very  high  temperature  of  the  filament,  all  gases 
except  the  inert  ones  may  be  removed  practically  quantitatively.  This  treatment  of 
the  lamp  to  improve  the  vacuum  resulted  in  a  marked  change  in  the  constants  of 
Richardson's  equation  and  also  changed  the  maximum  thermionic  currents. 

19 


To  improve  the  vacuum  still  further,  in  some  cases  the  entire  bulb  was  immersed  in 
liquid  air  and  the  filaments  were  again  heated  to  high  temperature  for  a  short  time. 
This  sometimes  resulted  in  a  further  change  in  the  thermionic  current. 

These  effects  wereiclearly  due  to  traces  of  residual  gas.  To  study  them  in  more  de- 
tail, some  experiments  were  carried  out  in  which  lamp  bulbs  containing  two  filaments 
(or  sometimes  three)  were  connected  to  a  vacuum  system  consisting  of  Topler  pump, 
sensitive  McLeod  gage  and  trap  immersed  in  liquid  air,  placed  directly  below  the  lamp. 
The  lamps  were  exhausted  to  less  than  0.0001  mm.  and  heated  for  one  hour  to  360  deg. 
C.,  to  drive  water  vapor  and  carbon  dioxide  into  the  liquid  air  trap.  No  stop-cocks 
were  used  in  the  entire  system,  so  that  vapors  of  vaseline,  etc.,  were  avoided.  The 
liquid  air  trap  prevented  the  entrance  of  mercury  vapor  into  the  lamp.  Care  was  taken 
to  keep  liquid  air  on  the  trap  day  and  night  during  the  whole  experiment. 

The  lamp  bulb  was 
about  3.5  cm.  diameter  and 
was  connected  to  the  rest 
of  the  vacuum  system  by  a 
tube  attached  at  the  top  of 
the  bulb  and  bent  into  a 
goose  neck.  In  this  .way 
the  bulb  could  be  immers- 
ed completely  in  liquid  air 
if  desired. 

After  exhaustion  of  the 
bulb  and  ageing  of  the 
filament  at  high  tempera- 
ture, a  run  was  made  with 
177  volts  on  the  anode,  but 
without  liquid  air  on  the 
bulb.  The  pressure,  ac- 
cording to  the  McLeod 
gage,  was  0.00012  mm. 
The  constants  of  Richardson's  equation  were  found  to  be 

a  =  22.106  amps,  per  sq.  cm., 
6  =  55,800  degrees. 

Liquid  air  was  now  placed  around  the  lamp  bulb,  and  the  thermonic  current  again 
determined.    During  this  run  the  pressure  was  constant  at  0.00007  mm. 
The  constants  were  then  found  to  be : 

a  =  34X106, 
6  =  55,500. 

A  plot  of  the  curve,  calculated  from  these  data  by  Richardson's  equation,  is  given  in 
Curve  I.,  Fig.  3,  together  with  the  experimentally  determined  points  (Curve  III). 

The  effect  of  cooling  the  bulb  in  liquid  air  was  thus  to  increase  the  thermionic  cur- 
rent considerably  (about  60  per  cent,  at  2000  deg.  K.).  The  filament  was  now  run  at 
2130  deg.  K.,  at  which  temperature  a  thermionic  current  of  0.00214  amp.  was 
observed.  The  liquid  air  was  then  removed  from  the  bulb.  The  thermionic  current 
fell  rapidly  to  0.00049  and  then  rose  to  0.00184,  at  which  it  remained  steady. 

Hydrogen. — Pure  hydrogen  to  a  pressure  of  0.012  mm.  was  now  admitted.  The 
thermionic  current  was  rather  variable,  changing  gradually  with  the  time,  but  by 


1800 


20 


running  the  temperature  up  and  down  several  times,  fairly  consistent  results  were 
obtained.     The  average  values  of  Richardson's  constants  were 

a -5.4X10", 

6  =  82,500. 

The  hydrogen  had  gradually  cleaned  up  during  these  runs  from  0.012  mm.  to  0.006 
mm.1  The  remainder  of  the  hydrogen  was  now  pumped  out  (down  to  0.0001 1  mm.)  and 
another  run  was  made  to  determine  the  thermionic  current  in  good  vacuum.  The  re- 
sults were 

a  =  4.3X1012, 

6  =  85,000. 

The  pressure  rose  during  this  run  from  0.00011  to  0.00052  mm. 
The  removal  of  the  hydrogen  thus  produced  relatively  little  effect,  and  certainly  did 
not  tend  to  cause  a  and  b  to  return  to  the  original  values  in  a  good  vacuum. 

More  hydrogen  (0.007mm.)  was  now  let  in,  and  the  constants  were  found  to  be 

a  =  7.6X1018, 
6  =  115,000. 

During  this  run  the  hydrogen  cleaned  up  from  0.007  to  0.004  mm. 

A  plot  of  the  curve  calculated  from  Richardson's  equation  is  given  in  Fig.  3  (Curve 
II),  together  with  the  experimentally  determined  points  (Curve  IV). 

The  general  effect  of  the  hydrogen  had  apparently  been  to  lower  permanently2  the 
thermionic  current,  especially  at  low  temperature.  It  had  at  the  same  time  increased 
the  value  of  the  constant  6  to  more  than  double  its  original  value. 

Effect  of  Bulb  Temperature. — At  this  stage  in  the  experiments  it  was  found  that 
touching  the  bulb  with  the  fingers  had  a  marked  effect  on  the  thermionic  current.  This 
was  due  to  a  temperature  effect.  With  the  filament  at  2190  deg.  K.  the  current  was 
0.0011  amp.  Warming  the  bulb  slightly  lowered  the  current  to  0.0003.  Placing  ice 
water  around  the  bulb  raised  the  current  to  0.0038.  Liquid  air,  however,  gave  the 
same  result  as  ice  water.  A  run  with  the  bulb  in  liquid  air,  with  a  pressure  of 
0.00025  mm.  of  hydrogen  in  the  bulb,  gave 

a  =  20.4X106, 
6  =  55,600. 

These  values  are  very  close  to  those  previously  obtained  with  liquid  air  before  any 
hydrogen  had  been  let  into  the  bulb. 

Immediately  after  this  run  a  beaker  containing  water  at  62  deg.  C.  was  placed 
around  the  bulb.  The  pressure  rose  to  0.0017  mm.  and  the  values  of  a  and  6  became 

a  =  7.7X1016, 
6=105,000. 

A  large  number  of  runs  were  now  made  at  different  bulb  temperatures.  The  results 
were  similar  to  those  already  given,  except  that  gradually  the  effect  of  heating  the  bulb 
became  less  marked  and  after  a  couple  of  days  practically  the  same  results  were  ob- 
tained with  the  bulb  at  50  deg.  as  at  0  deg.  For  example,  two  consecutive  runs  gave 

at    0°     a  =  54.106,     6  =  58,500; 
at  53°     a  =  60.106(     6  =  58,500. 

1  See  paper  on  Active  Modification  of  Hydrogen,  Langmuir,  J.  Amer.  Chem.  Soc.,  34,  1310  (1912). 

2  The  increase  in  the  value  of  6  in  Richardson's  equation  much  more  than  offsets  the  increase  in  a,  so  that  at  tempera- 
tures in  the  neighborhood  of  2000  deg.,  the  currents  in  hydrogen  are  very  much  smaller  than  the  original  currents  in 

vnrMitim 


vacuum. 


21 


Before  this  condition  of  insensitiveness  to  bulb  temperature  had  been  reached, 
tests  were  made  to  see  if  the  changes  in  the  thermionic  current  were  due  to  vacuum 
changes  or  absorption  of  gas  by  the  filament.  The  effect  of  interchanging  the  two  fila- 
ments was  tried  many  times.  That  is,  the  filament  which  had  previously  been  used  as 
anode  was  made  cathode  and  vice  versa.  In  no  case  did  this  change  make  any  material 
difference  in  the  magnitude  of  the  thermionic  current.  The  heating  of  the  bulb  pro- 
duced exactly  the  same  effect,  regardless  of  the  previous  history  of  the  filament.  The 
changes  that  did  occur  could  clearly  all  be  ascribed  to  vacuum  changes  caused  by  the 
absorption  or  evolution  of  gas  by  the  bulb. 

Water  Vapor. — The  fact  that  the  temperature  of  the  bulb  was  in  some  cases  so  much 
more  important  than  the  pressure  of  hydrogen  indicated  that  it  was  the  presence  of 
water  vapor  that  caused  the  decrease  in  the  thermionic  current  and  the  increase  in  b. 
To  test  this  out,  the  liquid  air  was  removed  for  a  couple  of  minutes  from  the  liquid  air 
trap  below  the  lamp  and  then  replaced.  The  effect  of  thus  allowing  water  vapor  to 
enter  the  lamp  was  to  make  the  thermionic  current  extremely  sensitive  to  the  tempera- 
ture of  the  bulb.  This  sensitiveness  could  be  destroyed  again  by  heating  the  bulb  to 
360  deg.  and  cooling. 

The  conclusion  to  be  drawn  from  these  facts  is  that  the  decrease  in  the  thermionic 
current  is  due  to  the  presence  of  traces  of  water  vapor.  The  McLeod  gage  gives  no  in- 
dication of  such  small  amounts  of  water  vapor,  but  the  fact  that  little  or  no  hydrogen 
is  evolved  by  the  action  of  the  filament  on  the  water  vapor,  together  with  the  fact  that 
the  water  vapor  can  remain  days  in  the  lamp  before  diffusing  down  into  the  liquid  air 
trap,  indicate  that  the  pressure  must  be  extremely  low — probably  not  over  10-6  mm. 
Yet  the  evidence  is  strong  that  such  pressures  of  water  vapor  have  an  enormous  effect 
on  the  saturation  thermionic  current  from  tungsten. 

Oxygen. — It  had  been  known  that  water  vapor  in  contact  with  a  hot  tungsten  fila- 
ment oxidizes  the  filament  with  the  liberation  of  atomic  hydrogen.  It  was,  therefore,  of 
interest  to  know  whether  the  marked  effect  produced  on  the  thermionic  current  by 
water  vapor  is  due  to  this  particular  reaction  or  whether  the  same  effect  will  not  be  pro- 
duced by  dry  oxygen. 

To  test  this  out,  the  system  was  exhausted  to  a  pressure  of  0.00012  mm.  The  mer- 
cury in  the  Topler  pump  bulb  was  raised  so  as  to  seal  off  the  bulb  and  pure  dry  oxygen 
was  admitted  to  the  bulb.  The  quantity  was  chosen  so  that  it  would  give  a  pressure  of 
0.005  mm.  when  allowed  to  flow  out  into  the  whole  system  by  lowering  the  mercury  in 
the  pump  bulb. 

The  filament  (A]  was  now  run  at  2190  deg.  K.,  and  the  other  filament  (B)  was 
charged  250  volts  positively  with'  respect  to  the  cathode.  The  thermionic  current 
wras  0.0031  amp. 

On  lowering  the  mercury  in  the  pump  bulb  and  allowing  the  oxygen  to  enter  the 
lamp  bulb,  the  thermionic  current  dropped  immediately  to  0.00013 ;  that  is,  to  4  per  cent 
of  its  original  value.  As  the  oxygen  gradually  disappeared,  the  current  steadily  rose 
in  value,  as  follows : 


Pressure,  Mm. 

Thermionic  Current. 

On  admitting  oxvgen         

0.005 

0.00013 

After     5  minutes                                     .    ... 

0.0003 

0.00030 

After  10  minutes 

0.00016 

O.OOOi-H) 

After  15  minutes  

0.00014 

0.00164 

After  20  minutes  

0.00010 

0.00230 

After  28  minutes  

0.00007 

0.00270 

22 


After  letting  in  another  supply  of  oxygen,  the  thermionic  current  was  determined  at 
various  temperatures  and  gave 

a  =  (),SX1013 
6  =  94.300. 

The  effect  of  oxygen  is  thus  found  to  be  quite  similar  to  that  of  water  vapor. 

Xitrogen. — Experiments  with  nitrogen  showed  that  this  gas  also  usually  decreased 
the  thermionic  current,  although  not  so  strongly  as  oxygen.  Since  nitrogen  does  not 
clean  up  as  rapidly  as  oxygen,  this  gas  was  chosen  for  a  series  of  experiments  to  de- 
termine whether  other  factors,  such  as  anode  voltage,  had  an  influence  on  the  Richard- 
son constants  in  the  presence  of  gas. 

Effect  of  Anode  Potential. — In  one  experiment  a  pressure  of  about  0.001  to  0.002  mm. 
of  nitrogen  was  present  in  the  bulb.  A  run  was  made  with  a  potential  of  220  volts  on 
the  anode,  then  a  run  with  100  volts,  and  then  another  run  at  220  volts.  The 
thermionic  currents  (milliamperes  per  sq.  cm.)  in  the  three  runs  were  as  follows: 

TABLE  II 


Temp. 

220  Volts. 

100  Volts. 

220  Volts. 

2045 

0.34 

0.29 

2090 

0.70 

0.63 

2140 

1.54 

1.29 

2190 

2.7 

4.0 

2.9 

2250 

6.3 

4.9 

7.0 

2325 

16.2 

5.0 

19.3 

2390 

21.0 

5.0 

20.0 

Pressure  of  N*  = 

0.0015  mm. 

0.0012  mm. 

0.0012  mm. 

These  results  show  that  under  certain  conditions  (low  temperature  and  proper 
pressure  of  nitrogen)  less  current  is  obtained  with  220  volts  than  with  100  volts.  In  this 
connection  it  may  be  said  that  these  thermionic  currents  were  reproducible  and  ac- 
curately measurable.  ^The  values  changed  gradually,  however,  as  the  nitrogen  cleaned 
up.1 

The  following  are  typical  runs  (Table  III)  selected  from  many  that  were  made 
while  the  nitrogen  was  gradually  disappearing : 

TABLE  III 


PRESSURE  0.00067  MM. 

PRESSURE  0.00011  MM. 

Temp. 

100  Volts. 

220  Volts. 

100  Volts.                         220  Volts. 

2045 

0.53 

0.36 

1.58 

1.50 

2090 

1.02 

0.86 

2.9 

2.90 

2140 

2.50 

1.90 

3Q 
.O 

6.2 

2190 

4.2 

4.50                            4.4 

9.8 

2250 

4.6 

11.8                               4.6 

11.4 

2325 

4.8 

13.8                  

11.8 

2390 

13.9 

12.1 

Thus,  as  the  nitrogen  cleans  up,  the  thermionic  current  increases  and  tends  to  re- 
turn to  its  original  value.  At  the  same  time  the  peculiar  effect  of  the  anode  voltage  in 
causing  less  current  to  flow  at  high  voltage  also  practically  disappears. 

1  This  is  the  electrochemical  clean-up  of  nitrogen  referred  to  by  the  writer  in  a  paper  on  "The  Clean-up  of  Nitrogen 
in  Tungsten  Lamps,"  Jour.  Amer.  Chem.  Soc.,  3o,  931  (1913). 


23 


All  the  experiments  with  nitrogen  were  made  with  the  bulb  surrounded  with  ice. 
This  precaution,  however,  did  not  seem  necessary  in  these  runs,  for  at  various  times 
the  ice  was  removed,  but  the  thermionic  current  remained  unchanged. 

In  order  to  investigate  in  more  detail  the  effect  of  the  anode  potential  at  various 
pressures  of  .nitrogen  and  different  filament  temperatures,  a  special  experiment  was 
undertaken.  A  lamp  containing  two  single  loop  filaments  in  a  large  bulb  was  sealed 
to  the  same  vacuum  system  and  exhausted  as  before.  The  filaments  were  of  0.0124  cm. 
diameter  and  each  8.9  cm.  long. 

After  ageing  the  filament,  the  thermionic  current  was  measured  at  120  and  230 
volts;  the  constants  a  and  b  were: 

at!20volts     a=1.2X1013,     6  =  83,000; 
230  volts     o  =  4.7X1012,     6  =  80,500. 

On  the  average,  even  with  currents  so  low  that  the  space  charge  should  have  no  ef- 
fect, the  thermionic  current  was  about  20  per  cent,  less  with  120  than  with  230  volts 
This  effect,  which  is  just  the  opposite  of  that  noted  in  the  preceding  experiment,  is  of 
fairly  common  occurrence  when  very  special  precautions  are  not  taken  to  avoid  traces 
of  certain  gases. 

The  effect  of  oxygen  on  the  thermionic  current  with  different  anode  potentials  was 
next  tried.  In  every  case  the  current  was  greatly  reduced  by  the  presence  of  this  gas. 
The  currents  obtained  with  120  volts  and  with  230  volts  were  always  practically 
identical. 

Another  measurement  of  the  thermionic  current  in  good  vacuum  (0.0001  mm.)  at  120 
and  240  volts  gave 

0=1.1X10", 
6  =  74,000. 

The  curve  obtained  by  Richardson's  equation  with  these  constants  is  given  in  Curve 
I,  Fig.  4,  while  Curves  II  and  III  are  drawn  through  the  experimentally  determined 
points. 

A  pressure  of  0.0021  mm.  of  nitrogen  was  now  introduced  and  the  following  meas- 
urements made  at  120  and  235  volts.  The  results  are  expressed  in  milliamperes  per 
sq.  cm. 

TABLE  IV 


PRESSURE  0.0021  MM.  NITROGEN 

Filament  Temp. 

Anode  120  Volts 

Anode  235  Volts 

2000 

0.132 

0.130 

2050 

0.278 

0.283 

2100 

0.515 

0.552 

2150 

1.07 

1.42 

2200 

2.90 

3.90 

2250 

6.4 

8.0 

2300 

12.9 

16.6 

2350 

>26 

>26. 

a    = 

1.66  X109 

2.2  X1010 

b    = 

68200 

73200 

These  results  are  plotted  (Curves  IV,  V,  Fig.  4)  for  comparison  with  the  preceding 
run  in  a  vacuum.  It  is  to  be  noted  that  at  temperatures  up  to  2050  deg.  the  thermionic 
current,  even  with  such  a  high  pressure  of  gas,  is  not  as  much  affected  by  the  anode 
voltage  as  it  had  previously  been  found  to  be  in  a  "  good  "  vacuum.  The  presence  of  the 

24 


nitrogen  has,  however,  entirely  removed  all  limitation  of  the  current  by  space  charge  at 
the  higher  filament  temperatures. 

Some  runs  were  now  made  with  the  filament  at  fixed  temperatures  while  the  anode 
voltage  was  varied  over  a  wide  range.  In  most  cases  the  pressure  of  nitrogen  was  kept 
as  nearly  constant  as  possible  at  0.0025  mm.;  in  some  runs,  however,  the  effects  pro- 
duced by  a  pressure  of  0.0010  mm.  were  studied. 

The  data  from  three  runs  with  the  filament  at  2100  deg.  are  given  in  Fig.  5.  The 
points  along  Curves  I  and  II  were  obtained  with  0.0025  and  0.0010  mm.  pressure  of 
nitrogen.  The  nitrogen  was  then  pumped  out  to  a  pressure  of  0.00016  mm.,  and 
the  points  along  Curve  III  were  then  obtained. 

These  curves  help  clear  up  several 
points  that  had  been  left  very  indefinite 
by  the  previous  data.  At  anode  poten- 
tials below  80  or  90  volts  the  effect  of 
nitrogen  is  evidently  to  increase  the 
thermionic  current  materially.  But 
above  a  certain  critical  potential,  which 
is  higher  the  lower  the  pressure  of 
nitrogen,  the  thermionic  current  de- 
creases as  the  anode  potential  is  raised. 
By  comparison  of  the  data  on  these 
curves  with  the  curves  of  Fig.  4,  it  is 
seen  that  they  are  entirely  consistent 
with  the  latter. 

In  looking  for  an  explanation  of  the 
shape  of  these  curves,  it  is  important  to 
bear  in  mind  that  the  lower  parts  of  the 
curves  are  determined  primarily  by  the 
space  charge.  When  the  thermionic 
current  is  plotted  against  temperature, 
as  in  Figs.  1  and  3,  the  lower  part  of 
the  curve  gives  the  saturation  thermionic 
current  (Richardson),  while  the  upper 
part  gives  the  part  which  is  limited  by 
the  space  charge.  On  the  other  hand, 

when  we  plot  the  current  against  the  anode  potential,  as  in  this  case,  the  two  parts  are 
interchanged  in  position ;  thus  the  lower  part  of  the  curve  gives  the  current  as  limited 
by  space  charge  and  the  upper  horizontal  part  gives  the  saturation  current. 

The  lower  part  of  Curve  III,  if  obtained  in  a  perfect  vacuum,  should,  therefore,  fol- 
low equation  (11) :  in  other  words,  the  current  should  increase  with  Fl  By  plotting 
the  first  six  points  of  this  curve  on  logarithmic  paper,  a  straight  line  was  obtained,  but 
the  slope,  instead  of  giving  3/2  as  the  exponent  of  V,  gave  1.71.  This  difference  is  cer- 
tainly due  to  residual  gas.  Curve  IV,  Fig.  5,  was  obtained  by  continuing  the  curve 

i  =  constant  X  71'71. 

The  Curve  III  separates  from  IV  above  125  volts  because  the  current  gradually 
reaches  saturation  for  the  filament  at  2100  deg. 

We  are  now  in  a  position  to  discuss  the  Curve  I  and  II.  At  anode  potentials  below 
20  volts  the  curves  seem  to  coincide  fairly  well,  but  the  current  rapidly  increases  at 


2200 


2300'K. 


Fig.  4 


25 


J3ff 


higher  potentials.  Plotting  the  first  six  points  of  Curve  I  on  logarithmic  paper  does 
not  give  a  straight  line,  but  the  exponent  of  V  is  found  to  increase  from  about  2  to  over 
().  The  cause  of  this  rise  in  current  is  probably  that  positive  ions  are  formed  by  the 
collisions  of  the  electrons  with  nitrogen  molecules.  The  positive  ions  moving  slowly 
carry  only  a  very  minute  fraction  of  the  current,  yet  by  their  mere  presence  they 
materially  reduce  the  space  charge  and  therefore  allow  a  larger  current  to  flow. 

If  this  were  the  only  factor,  one  would  expect  with  increasing  voltage  that  the  cur- 
rent would  rise  to  the  normal  saturation  current  and  then  remain  constant  until  the 
voltage  reaches  a  point  where  additional  electrons  are  liberated  from  the  cathode  by 
the  impact  of  positive  ions  against  it.    But  before  this  point  is  reached,  some  other  fac- 
tor begins  to  make  itself  felt.    This  is 
evidently  a  limitation  of  the  current,  not 
by  space  charge,  but  by  some  phenom- 
enon   which    prevents    the    emission   of 
electrons  from  the  cathode.     It  is,  how- 
ever, not  due  to  a  simple  alteration  cf 
the  properties  of  the  material    of  the 
cathode,  for  its  magnitude  depends  on 
the  anode  potenial. 

The  following  theory  seems  to  be 
consistent  with  all  the  observed  facts 
and  may  prove  to  be  the  correct  expla- 
nation of  the  phenomenon. 

Theory  of  the  Effect  of  Nitrogen  on  the 
Thermionic  Current.  —  The  writer  has 
shown1  that  nitrogen  does  not  react 
perceptibly  with  solid  tungsten  at  any 
temperature,  but  does  react  completely 
with  all  the  tungsten  that  evaporates 
from  the  filament  to  form  the  compound 
WNZ.  The  evidence  indicated  that  this 
compound  is  unstable  at  temperatures 
Fig.  5  above  2400  deg.  Now  although  ordinary 

nitrogen  does  not  react'.with  solid  tung- 
sten to  form  a  compound,  it  is  not  improbable  that  nitrogen  ions"  possessing  enor- 
mously high  kinetic  energy  as  compared  with  the  ordinary  molecules,"will  do^so.  The 
compound  formed,  however,  being  unstable,  does  not  permanently  remain  on  the  sur- 
face, but  either  decomposes  or  volatilizes.  Any  such  process,  however,  requires  a  cer- 
tain amount  of  time,  so  that  the  molecules  would  remain  on -or  in  the  surface  during 
perhaps  a  perceptible  fraction  of  a  second.  The  higher  the  temperature  of  the  filament 
the  shorter  the  average  time  that  would  elapse  between  the  formation  of  a  molecule 
of  the  compound  and  its  elimination  from  the  surface. 

Let  us  now  apply  this  theory  to  the  data  presented  in  Fig.  5.  At  an  anode  potential 
below  20  volts,  no  positive  ions  are  formed,  so  the  surface  of  the  tungsten  is  not  ex- 
posed to  bombardment.  At  higher  voltages  the  positive  ions  are  produced  in  increasing 
numbers  and  strike  the  cathode  with  increasing  velocity.  As  a  result  the  surface  be- 
comes more  or  less  completely  covered  by  a  layer  of  molecules  of  the  compound. 
Since  as  a  rule  a  compound  would  be  expected  to  emit  electrons  less  freely  than  a  metal, 

1  Jour.  Amer.  Chem.  Soc.,  35,  p.  943  (1913). 


20 


it  is  not  unreasonable  to  imagine  this  surface  layer  as  being  the  cause  of  the  decreased 
electron  emission. 

In  a  perfect  vacuum  the  actual  electron  emission  from  the  tungsten  is  independent 
of  the  anode  voltage  as  indicated  in  Curve  III',  Fig.  5.  However,  at  low  anode  voltage? 
the  space  charge  causes  most  of  the  electrons  to  return  to  the  filament,  so  that  the  actual 
current  obtained  is  as  shown  in  Curve  III.  On  the  other  hand,  in  low  pressures  of  ni- 
trogen the  actual  electron  emission  decreases  as  the  anode  potential  increases,  as  illus- 
trated by  the  hypothetical  Curve  I'  and  II'.  As  in  the  case  of  the  vacuum,  however,  at 
low  voltages  the  space  charge  causes  the  return  of  most  of  the  electrons  to  the  cathode, 
so  that  the  actual  curves  (I  and  II)  show  a  current  which  rises  rapidly  with  increasing 
potentials.  When  the  potential  becomes  sufficient  to  prevent  the  return  of  any 
electrons  to  the  cathode, 
then  the  current  becomes 
limited  solely  by  the  elec- 
tron emission  from  the 
metal.  At  high  voltages, 
therefore,  the  current  de- 
creases with  increased 
anode  potential  because 
of  the  increasing  propor- 
tion of  the  cathode  surface 
covered  with  the  com- 
pound. 

The  theory  thus  ac- 
counts for  the  shape  of  the 
curves  in  Fig.  5  in  a  satis- 
factory way.  It  also  gives 
a  reason  for  the  shape  of 
the  curves  obtained  with 
nitrogen,  given  in  Fig.  4. 

The  effect  of  nitrogen  has  invariably  been  to  decrease  greatly  the  saturation  current  at 
240  volts.  At  low  temperatures  this  effect  is  much  more  pronounced  than  at  high. 
This  is  indicated  by  the  fact  that  the  introduction  of  nitrogen  (or  oxygen)  always  in- 
creases the  value  of  Richardson's  constant  b  from  55,000  up  to  80,000  or  more.  These 
facts  are  in  complete  accord  with  the  theory,  for,  as  has  been  pointed  out,  the  length  of 
time  that  the  molecules  of  the  compound  will  remain  on  the  metal  would  be  much  less 
at  high  temperatures.  Therefore,  the  higher  the  temperature,  the  smaller  the  pro- 
portion of  the  surface  covered  by  molecules  of  the  compound,  and  the  nearer  the  ob- 
served thermionic  current  approaches  the  normal  saturation  current. 

Whatever  the  nature  of  change  in  the  surface  of  the  tungsten  which  decreases  the 
electron  emission,  it  is  one  which  does  not  persist  for  more  than  a  few  seconds  after  the 
removal  of  the  nitrogen.  In  the  course  of  all  these  experiments,  except  before  ageing 
the  filaments,  there  were  never  any  effects  which  could  not  be  immediately  duplicated 
after  interchanging  the  functions  of  the  two  electrodes.  There  is,  therefore,  no  reason 
to  call  upon  any  "inexhaustible  supply  of  gas"  in  the  filament  to  account  for  any 
of  the  phenomena  observed.1 


Fig.  6 


1  The  writer  feels  strongly  that  the  majority  of  the  cases  cited  in  the  literature  where  fine  platinum  wires,  etc.,  ap- 
parently continue  to  give  off  gas  after  prolonged  heating,  are  caused  not  by  gas  from  the  wire,  but  by  water  vapor  or  other 
gases  liberated  from  the  walls  (or  vapors  from  stopcock  grease  or  sealing-wax)  which  are  changed  chemically  by  the  hot 
wire  or  by  electrical  discharges. 


27 


Effect  of  Anode  Potential  at  Higher  Filament  Temperature.  —  The  data  on  the 
effect  of  anode  potential  obtained  with  the  filament  at  a  higher  temperature, 
2300  deg.  K.,  is  given  in  Fig.  6.  With  0.0010  mm.  of  nitrogen,  the  current  rose  steadily 
(Curve  I)  until  a  potential  of  about  135  volts  was  reached.  With  potentials  higher 
than  this,  the  current  would  rise  to  a  high  value,  0.013  amp.  per  sq.  cm.  or  more, 
immediately  on  lighting  the  filament,  and  the  discharge  was  accompanied  by  a 
strong  purple  glow  filling  the  bulb.  Suddenly  the  current  fell  to  0.005  amp.  per  sq. 
cm.  "or  less,  and  at  the  same  time  the  purple  glow  vanished.  Every  time  that  this 
happened  the  pressure  in  the  system  would  fall  from  about  0.0012  to  0.0006  or 
less,  so  that  fresh  nitrogen  had  to  be  admitted  after  each  trial. 

With  a  pressure  of 
about  0.0025  mm.  of  ni- 
trogen (Curve  II)  a  dis- 
continuity occurred  in 
the  current  values  at  an 
anode  potential  of  70 
volts.  Above  this,  how- 
ever, the  current  was 
again  steady.  These  un- 
stable conditions  are 
probably  due  to  ioniza- 
tion  reaching  such  a 
value  that  the  bombard- 
ment of  the  cathode  by 
the  positive  ions  gives 
rise  to  additional  elec- 
trons. These  in  turn 
cause  the  formation  of 
fresh  positive  ions.  The 

effect  is  thus  one  which  can  readily  become  unstable.  It  is  interesting  to  note, 
however,  that  even  with  this  additional  electron  emission,  the  current  is  still  less  than 
the  saturation  current  in  a  perfect  vacuum,  which  from  Curve  I',  Fig.  4,  would  be  about 
0.050  ampere  per  sq.  cm.  Undoubtedly  at  higher  pressures  and  voltages  than  those 
used  the  currents  caused  by  ionization  would  ultimately  greatly  exceed  the  saturation 
current  from  the  filament. 

Curve  III  gives  the  current  obtained  with  a  much  better  vacuum  and  prob- 
ably represents  very  closely  the  normal  current  as  limited  by  space  charge.  The 
effect  of  the  nitrogen  is  thus  mainly  to  produce  positive  ions  and  neutralize  the  space 
charge. 

Argon. — A  series  of  experiments  was  made  to  determine  the  thermionic  current  in 
low  pressures  of  argon.  The  surprising  result  was  obtained  that  the  saturation  cur- 
rents were  in  every  case  (pressures  up  to  0.002  mm.)  identical  with  the  results  previously 
obtained  in  the  best  vacuum.  That  the  argon  had  the  further  effect  of  neutralizing  the 
space  charge  is  shown  by  the  relatively  large  currents  obtained  with  anode  voltages  of 
only  40-100  volts. 

In  all,  about  thirty  runs  were  made,  and  with  two  exceptions  they  all  gave  values  of 
the  Richardson  constant  b  between  50,500  and  58,000.  The  Curves  I  and  II,  Fig.  7, 
are  examples  of  typical  runs.  It  is  seen  by  comparison  with  Fig.  3  that  the  maximum 
currents  obtained  are  considerably  larger  than  those  to  be  had  in  vacuum  with  similar 


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Fig.  7 

28 


voltages,  but  these  larger  currents  are  due  solely  to  the  removal  of  the  limitation  im- 
posed by  the  space  charge.  At  lower  temperatures  the  currents  obtained  are  practically 
identical  with  those  in  vacuum. 

Another  remarkable  fact  about  the  effect  of  argon  on  the  thermionic  current  is  that 
considerable  admixtures  of  nitrogen  or  even  oxygen  have  little  or  no  effect.  Thus, 
while  the  filament  was  running  at  2190  deg.  in  argon  at  0.0016  mm.  pressure,  an 
amount  of  nitrogen  was  let  in,  which  raised  the  pressure  to  0.0035,  yet  the  current  at 
240  volts  changed  only  about  5  per  cent. 

In  another  case,  while  the  filament  was  running  under  similar  conditions  as  in  the 
test  with  nitrogen,  an  amount  of  oxygen  was  suddenly  admitted  sufficient  to  raise 
the  total  pressure  from  0.0016  to  0.0035  mm.  In  this  case  the  current  was  decreased  by 
about  10  per  cent  for  a  few  seconds,  but  rapidly  returned  within  a  couple  of  per 
cent  of  the  original  value.  Upon  lowering  the  temperature  to  2045  deg.,  the 
thermionic  current  was  found  to  be  only  7  per  cent  of  its  value  in  argon,  while  at 
2140  deg.  it  was  20  per  cent  of  its  original  value.  If  these  results  are  compared  with 
those  cited  previously  on  the  effect  of  oxygen  on  the  thermionic  current,  it  will  be  seen 
that  the  argon  has  enormously  weakened  the  effect  produced  by  oxygen,  especially 
at  higher  temperatures. 

Argon  acted  remarkably  in  another  respect.  The  thermionic  current  in  argon 
caused  very  marked  disintegration  of  the  hot  cathode;  whereas  this  effect  is  entirely 
absent  in  a  good  vacuum  with  a  pure  electron  current,  and  only  present  to  a  very  slight 
degree  in  pressures  of  nitrogen  as  low  as  0.002  mm.  With  the  argon  the  filament  rapidly 
increased  in  resistance  by  loss  of  material  which  deposited  on  the  bulb  in  the  form  of 
black  bands,  principally  behind  the  anode.  These  bands  had  more  or  less  the  shape  of 
the  anode  filament  and  had  a  white  strip  down  their  centers — evidently  the  shadow 
cast  by  the  anode.  This  proves  that  the  tungsten  which  was  sputtered  from  the 
cathode  was  or  became  negatively  charged.  It  is  surprising  that  the  thermionic  current 
was  identical  with  that  in  a  high  vacuum,  notwithstanding  this  marked  disintegration 
of  the  cathode.  This  experiment  certainly  proves  that  there  is  no  necessary  relation 
between  cathode  disintegration  and  thermionic  current. 

These  results  with  argon  are  strong  support  for  the  theory  that  the  positive  ions  of 
nitrogen  or  ordinary  molecules  of  oxygen  form  unstable  compounds  with  the  tungsten 
which  prevent  the  normal  electron  emission.  Argon  not  being  capable  of  reacting 
chemically  with  the  tungsten  does  not  reduce  the  thermionic  currents. 

The  explanation  of  the  action  of  argon  in  preventing  oxygen  and  nitrogen  from  hav- 
ing their  normal  effect  is  probably  that  the  bombardment  of  the  cathode  by  the  posi- 
tive argon  ions,  which  is  undoubtedly  (in  accordance  with  Stark 's  theory)  the  cause  of 
the  sputtering  of  the  cathode,  also  sputters  away  any  compound  formed  and  thus 
keeps  the  surface  of  the  tungsten  clean.  At  lower  temperatures  and  hence  lower  cur- 
rents, the  sputtering  is  less  marked  and  therefore  the  argon  interferes  less  with  the 
normal  action  of  the  oxygen. 

EXPERIMENTS  WITH  PLATES  AND  CYLINDERS  AS  ANODES 

In  all  the  experiments  mentioned  thus  far,  the  anode  has  been  a  tungsten  filament 
which  has  been  freed  from  gas  by  heating  to  2500  deg.  Several  experiments,  however, 
were  also  made  with  anodes  of  thin  sheet  metal,  in  the  form  of  plates  or  cylinders.  In 
general,  in  these  experiments,  unless  very  special  methods  of  treating  the  electrodes  are 
adopted,  the  evidences  of  the  presence  of  gas  are  much  more  marked  than  in  the  ex- 
periments with  filaments  as  anodes.  A  great  variety  of  erratic  effects  occur,  such  as 

29 


gradual  changes  in  the  thermionic  current,  and  various  kinds  of  fatigue  effects.  Some 
of  these  effects  are  particularly  pronounced  at  low  temperatures  of  the  filament.  A 
large  amount  of  data  has  been  obtained  in  studying  these  effects,  and  much  of  it  is  of 
such  interest  that  the  results  will  be  published  in  detail  in  subsequent  papers.  For 
the  present  it  will  suffice  to  consider  the  results  of  a  few  runs  at  various  filament  tem- 
peratures and  anode  voltages,  in  a  lamp  containing  a  cylindrical  anode. 

The  anode  in  this  experiment  consisted  of  a  cylinder  of  platinum  foil,  3.5  cm.  dia- 
meter and  6  cm.  long.  Inside  of  this,  about  1  cm.  apart,  were  placed  two  single  loop 
tungsten  filaments  of  wire  0.0127  cm.  diameter  and  each  9.9  cm.  long.  The  surface  of 
each  was  thus  0.40  sq.  cm.  A  cylindrical  glass  bulb  fitted  closely  about  the  platinum 
cylinder. 

The  platinum  cylinder,  before  placing  in  the  lamp  bulb,  was  ignited  for  a  few 
minutes  over  a  blast  lamp  to  a  white  heat  and  washed  with  nitric  acid,  but,  other 
than  this,  purposely  not  subjected  to  special  treatment. 

This  lamp  was  sealed  to  the  same  system  as  before  and  the  same  care  was  used  in 
exhausting  it  as  in  the  other  experiments.  That  is,  after  exhausting  to  0.001  mm.,  the 
bulb  was  heated  to  370  deg.  C.  for  an  hour  and  a  half.  From  the  beginning  to  the  end 
of  the  experiment  the  trap  directly  below  the  lamp  was  kept  in  liquid  air. 

During  the  first  few  days  the  results  were  extremely  erratic  and  were  characterized 
especially  by  lag  or  fatigue  effects.  The  results  showed  clearly,  however,  that  these 
were  not  due  to  gas  contained  in  the  filament,  but  were  rather  caused  by  gas  from  the 
anode  liberated  by  electron  bombardment.  The  quantities  of  gas  liberated,  however, 
were  very  small  so  that  even  with  the  Topler  pump  no  great  difficulty  was  ex- 
perienced in  keeping  the  pressure  continually  below  0.001  mm. 

The  runs  about  to  be  described  were  made  on  the  tenth  day  after  the  beginning  of 
the  experiment.  By  this  time  most  of  the  erratic  effects  had  disappeared  and  the  re- 
sults obtained  were  beautifully  reproducible. 

A  series  of  runs  was  made  with  the  following  anode  voltages,  and  in  the  following 
order:  240,  124,  30,  50,  70,  90,  70,  50,  30,  20,  110,  124.  At  each  voltage  the  therm- 
ionic currents  were  measured  at  temperatures  from  1850  deg.  to  2350  deg.  or  2500 
deg.,  in  steps  of  50  deg.  The  pressure  during  all  these  runs  varied  within  the  limits 
0.00007  to  0.00023  mm.  In  every  case  different  runs  at  the  same  anode  voltage  gave 
practically  identical  results.  Furthermore,  at  temperatures  so  low  that  saturation 
current  was  obtained,  the  thermionic  currents  at  different  voltages  were  the  same. 
The  results  of  these  runs  are  given  in  Fig.  8.  All  the  experimentally  determined  points 
are  given,  except  where  the  curves  run  together,  and  then  the  points  for  the  curve  made 
at  110  volts  are  given. 

For  comparison  writh  these  results,  the  Curve  I,  of  Fig.  3,  has  been  replotted  in  Fig. 
S  (continuous  heavy  line).  This  curve  gives  the  results  that  were  obtained  under 
good  conditions  in  a  vacuum  and  in  a  case  in  which  temperatures  were  determined 
in  the  same  manner  as  in  the  present  experiment. 

The  upper  portion  of  the  curve  (not  given  in  Fig.  3)  was  calculated  by  Richardson's 
equation,  using  the  constants 

a  =  34  X  1 06, 
6  =  55,500. 

It  will  be  seen,  from  Fig.  8,  that  the  currents  obtained  with  the  cylindrical  anode  are 
much  larger  than  any  recorded  previously  in  this  paper.  Thus  with  110  volts  or  more, 
currents  up  to  0.350  ampere  per  sq.  cm.  (actual  current  measured  was  0.139  amp.) 
were  obtained.  But  the  curves  show  plainly  that  at  the  same  temperature  of  the  fila- 

30 


ment,  the  currents  are  always  less  than  those  obtained  under  the  best  vacuum  condi- 
tions. The  shape  of  the  anode  reduces  the  effect  of  space  charge  and  thus  allows  much 
more  current  to  flow  with  the  same  voltage  than  in  the  previous  experiments. 

This  experiment  offers  the  most  convincing  evidence  possible  of  the  correctness  of 
the  general  theory  of  the  effects  of  gas  outlined  previously. 

According  to  this  theory,  the  effect  of  certain  gases  (probably  most  gases  except  the 
inert  gases)  is  to  cut  down  the  normal  electron  emission  from  the  heated  metal,  by  the 
formation  of  an  unstable  compound  on  the  surface.  At  higher  temperatures  the  rate  of 
decomposition  or  evaporation  of  the  compound  is  greater  and  hence  fewer  molecules 
of  the  compound  remain  on  the  surface.  At  sufficiently  high  temperatures  the  com- 
pound should  completely  disappear  and  thus  allow  the  electron  emission  to  become 
normal. 

The  present  experiment  is  in  full  accord  with  this  theory.  At  low  temperatures 
the  electron  emission  (satu- 
ration current)  is  much  less 
than  the  normal,  but  at 
higher  temperatures  it  in- 
creases rapidly  up  to  the 
normal  value,  but  the  cur- 
rent never  exceeds  the  normal 
current. 

These  facts  are  made 
much  clearer  by  plotting 
log  i/\/T  against  1/7  (Fig. 
9).  With  these  functions 
as  coordinates,  the  points 
should  lie  on  a  straight  line 
if  the  current  follows  Rich- 
ardson's equation.  Curve 
I  is  the  normal  vacuum 
curve  shown  in  Figs.  3  and  Fig.  s 

S,    and   Curve   II  is  that 

obtained  from  the  experiment  with  the  cylindrical  anode  at  a  potential  of  110  volts. 
It  is  seen  that  the  second  line  is  not  straight.  The  lower  part  is  practically  straight, 
but  the  upper  part  does  not  cross  the  normal  vacuum  curve,  but  instead  bends  over 
and  joins  it.  The  reason  that  it  does  not  follow  it  at  higher  temperatuies,  is  probably 
that  space  charge  is  having  some  effect,  as  in  the  run  with  90  volts  (see  Fig.  8). 
The  extremely  close  coincidence  between  the  observed,  and  calculated  curves  over 
the  short  range  of  contact  is  undoubtedly  partly  accidental. 

The  fact  that  the  curves  obtained  at  low  anode  voltages  (20-30  volts)  coincide  at 
lower  temperatures  with  the  curves  obtained  with  high  potentials,  shows  in  this  case 
that  the  compound  on  the  surface  is  not  formed  from  positive  ions,  but  is  formed 
directly  by  a  reaction  between  the  gas  and  the  metal.  The  gas  in  this  case  is  probably 
carbon  monoxide  or  hydrogen,  and  can  easily  be  supposed  to  react  in  this  way  to  form 
unstable  compounds.  With  nitrogen,  on  the  other  hand,  the  evidence  is  good  that 
the  compound  forms  only  when  positive  nitrogen  ions  strike  the  filament. 

The  effect  of  the  gas  in  eliminating  the  space  charge  is  strikingly  shown  in  this  ex- 
periment. At  higher  potentials  the  thermionic  current  increases  much  more  rapidly 
than  the  three  halves  power  of  the  voltage.  This  shows  that  positive  ions  are  formed 


2100 


2200 


2300 


2400 


2500 


2600*K 


31 


which  reduce  or  eliminate  the  space  charge  produced  by  the  electrons,  yet  themselves 
do  not  carry  a  perceptible  portion  of  current. 

Some  other  experiments,  to  be  described  in  detail  in  a  subsequent  paper,  have 
shown  that  the  bombardment  of  the  cathode,  even  by  positive  ions  of  a  velocity  cor- 
responding to  only  110  volts,  causes  the  cathode  to  emit  electrons  having  a  velocity  cor- 
responding to  6-12  volts.  Thus  a  third  filament  in  a  bulb  in  which  a  thermionic 
current  is  flowing  often  charges  up  rapidly  to  a  potential  of  10  volts  negative  with 
respect  to  the  negative  end  of  the  cathode  filament.  This  observation  is  entirely  in  line 
with  previous  observations  on  delta  rays  and  electron  emission  caused  by  canal  rays. 

GENERAL  DISCUSSION 

The  evidence  presented  in  this  paper  and  the  theories  that  have  been  advanced  are 

thought  to  throw  a  rather 
new  light  on  the  elec- 
tron emission  from 


-4 


"It 


-5 


-6. 


V<N 


<; 


41         4Z        43        44        45 


46         47 
?ig.  9 


in- 
candescent solids  in  high 
vacuum. 

In  the  first  place,  the 
work  indicates  that  the 
experimental  conditions 
that  have  commonly  been 
employed  in  the  investi- 
gation of  thermionic  cur- 
rents in  vacuo  have  not 
been  well  adapted  to  elim- 
inate important  secondary 
effects.  The  proper  con- 
ditions seem  to  be : 

1.  Extremely  high 
vacuum;  that  is,  a  pres- 
sure below  0.0001  mm.  should  be  obtained.  The  presence  of  certain  gases  is  much 
more  injurious  than  others.  Gases  such  as  oxygen,  water  vapor,  carbon  dioxide,  and 
hydro-carbons,  which  are  very  active  chemically  at  high  temperatures,  should  be 
especially  avoided.  This  means  that  all  stopcocks  and  sealing-wax  must  be  eliminated 
and  all  glass  parts  not  to  be  cooled  by  liquid  air  must  be  heated  for  at  least  an  hour 
to  360  deg.  or  more.  Even  with  the  Gaede  molecular  pump  these  precautions  are 
necessary. 

2.  Avoidance  of  large  anodes,  except  those  that  have  been  especially  treated  by 
heating  in  a  vacuum  to  2000  deg.  or  have  been  exposed  to  powerful  electron  bom- 
bardment in  a  very  high  vacuum.    Treating  the  metal  by  making  it  an  electrode  in  an 
ordinary  glow  discharge,  except  when  the  inert  gases  are  used,  is  about  the  worst  thing 
that  could  be  done  to  it.    Preferably,  the  anode  should  consist  of  tungsten  wire  which 
is  freed  from  gas  by  heating  to  2500  deg.  for  ten  minutes.    This  should  be  done  in  the 
apparatus  itself. 

3.  The  relative  position  and  size  of  the  electrodes  should  be  such  that  space  charge 
does  not  limit  the  current  to  an  undesirable  degree. 

If  proper  precautions  are  taken  to  obtain  an  extremely  high  vacuum,  and  if  the 
anode  consists  of  molybdenum  or  tungsten  and  is  given  a  preliminary  treatment  by 
exposing  to  very  powerful  electron  bombardment,  it  is  possible  to  use  cylindrical  anodes 
without  obtaining  the  slightest  evidence  of  positive  ionization.  For  this  purpose  the 


32 


anode  should  be  charged  to  several  thousand  volts  and  the  filaments  raised  to  such 
temperatures  that  50-200  milliamperes  thermionic  current  are  obtained.  Under  these 
conditions  the  anode  becomes  heated  to  a  bright  red  or  white  heat,  and  the  combined 
effect  of  the  electron  bambardment  and  the  high  temperature  is  to  free  the  anode  from 
gas.  After  such  treatment  pure  electron  currents  of  several  tenths  of  an  ampere  may 
be  obtained  without  positive  ionization.  Dr.  Dushman  has  found  that  under  these 
conditions  the  space  charge  equation  (20)  holds  with  a  high  degree  of  accuracy.1  In 
this  way,  by  using  three  cylindrical  anodes  on  the  guard-ring  principle,  it  should  be 
possible  to  use  equation  (20)  to  determine  the  value  of  e/m  with  a  degree  of  precision 
greater  than  that  obtainable  by  any  other  method.  Neither  the  writer  nor  Dr.  Dush- 
man intends  to  make  such  precision  measurements  and,  therefore,  we  should  like  to  sug- 
gest that  this  method  be  seriously  considered  by  those  who  plan  to  make  such  deter- 
minations. 

Failure  to  observe  the  conditions  given  above  has,  it  is  feared  by4;he  writer,  very 
seriously  vitiated  most  of  the  quantitative  results  obtained  in  the  past  on  thermionic 
current  in  a  vacuum.  It  is  also  undoubtedly  the  principal  cause  of  the  opinion  which 
is  so  prevalent  today  that  the  electron  emission  from  hot  solids  is  a  secondary  effect, 
probably  usually  produced  by  chemical  reactions,  which  would  disappear  if  a  perfect 
vacuum  could  be  obtained. 

The  evidence  presented  in  this  paper  will,  it  is  hoped,  counteract  these  unfortunate 
tendencies  and  help  place  the  Richardson  theory  of  electron  emission  on  a  firm  footing 
or  at  least  stimulate  the  critical  study  of  the  theory.  The  thermionic  effect  is  of  at 
least  as  great  intrinsic  interest  as  the  photoelectric  effect,  and  should  receive  as  much, 
if  not  more,  attention  on  the  part  of  physicists. 

Let  us  now  examine  more  closely  some  of  the  experiments  which  have  led  to  the 
common  knowledge  of  thermionic  currents  in  vacuo. 

With  platinum  wires  extremely  variable  results  have  been  obtained.  H.  A.  Wilson 
found  that  by  heating  the  wire  in  oxygen  or  by  previously  boiling  it  for  24  hours  in  nitric 
acid,  the  thermionic  current  would  be  reduced  to  the  100,000th  part  of  its  original  value. 
This  lower  value,  however,  he  considers  the  normal  value  in  a  vacuum,  and  believes 
the  increase  observed  when  hydrogen  is  admitted  to  be  due  to  some  secondary  effect. 

The  effect  of  oxygen  on  the  thermionic  current  from  platinum  is  so  strikingly 
similar  to  that  observed  with  tungsten  that  the  writer  can  not  help  but  feel  that  the 
cause  in  both  cases  is  similar.  The  writer  has  found2,  and  will  soon  publish  his  results 
in  detail,  that  when  a  platinum  wire  is  heated  in  oxygen  at  low  pressure,  the  platinum 
evaporates  at  the  same  rate  as  in  a  vacuum  and  that  the  platinum  vapor  combines 
quantitatively  with  the  oxygen  after  it  leaves  the  surface  of  the  wire  to  form  the 
compound  PtCV  This  is  identically  the  same  type  of  reaction  that  has  been  observed 
«vith  tungsten  and  nitrogen.  This  suggests  strongly  that  oxygen  would  have  a  similar 
effect  on  the  thermionic  current  from  platinum  that  nitrogen  has  on  tungsten.  In  other 
words,  the  oxygen  would  cut  the  thermionic  current  down  to  a  value  lower  than  the 
normal  vacuum  current.  Hydrogen  would  reduce  the  oxide  and  allow  the  normal 
current  to  flow.  Of  course  it  is  quite  possible,  although  not  probable,  that  some 
gases  may  increase  the  thermionic  current  instead  of  decreasing  it. 

PRING  AND  PARKER'S  EXPERIMENTS3 

The  excellent  experiments  of  these  investigators  have  been  perhaps  the  most  con- 
vincing evidence  that  the  thermionic  electron  emission  is  a  secondary  effect.  Let  us, 
therefore,  criticize  the  experiment  in  the  light  of  the  new  theory. 

1  These  results  will  soon  be  published  by  Dr.  Dushman. 
-   Tour.  Amer.  Chem.  Soc..  So,  p.  944  (1913). 
3  Phil.  Ma«.,  23.  192  (1912). 


In  the  first  place,  carbon  is  a  substance  which  at  high  temperatures  is  particularly 
active  chemically,  so  that  it  probably  reacts  with  every  gas  present  in  the  system.  The 
residual  gases  may,  therefore,  be  expected  to  have  a  particularly  strong  action  in  re- 
ducing (or  possibly  increasing)  the  thermionic  current.  Although  Pring  and  Parker 
have  attempted  to  obtain  a  high  vacuum,  and  have  measured  the  pressures,  yet  the 
best  vacuum  they  claim  to  have  attained,  while  heating  the  cathode,  is  0.001  mm. 
Actually,  however,  the  pressure  must  have  been  at  least  several  times  this  amount,  for 
mercury  vapor  (0.002  mm.)  had  free  access  to  the  apparatus  and  they  used  "soft  wax" 
in  several  places,  which  gives  off  large  quantities  of  various  vapors,  all  of  which  are 
readily  condensible  and  therefore  not  indicated  by  the  McLeod  gauge. 

In  the  second  place,  the  distance  between  anode  and  cathode  in  their  experiments 
varied  from  4.8  to  1 1  cm.,  so  that  in  a  perfect  vacuum,  with  only  330  volts  on  the  anode 
they  could  have  obtained  only  very  little  current  because  of  the  space  charge.  For  ex- 
ample, if  we  calculate  from  equation  (11)  the  total  current  that  could  be  carred  at  330 
volts  between  two  electrodes  8.0  cm.  in  area  at  a  distance  of  4.8  cm.,  we  obtain  only 
0.0048  ampere.  Whereas  Pring  and  Parker  obtained  currents  as  high  as  40  amperes, 
with  only  40  to  GO  volts  in  the  beginning;  later  on,  after  the  carbon  had  ceased  giving  off 
large  quantities  of  gas,  the  currents  fell  to  as  low  as  0.000016,  with  the  temperature  of 
the  carbon  rod  at  2050  deg. 

The  reason  that  the  currents  in  their  experiments  went  down  to  values  so  much  be- 
low that  which  we  have  calculated  above  from  the  "space  charge"  equation  may  pos- 
sibly be  that  the  electron  emission  was  actually  decreased  to  that  low  value  by  the 
presence  of  gas.  A  much  more  probable  explanation  suggests  itself  by  referring  to  the 
diagram  of  their  apparatus  (Fig.  1,  Pring  and  Parker's  paper).  Apparently  they  have 
placed  a  glass  cylinder,  F,  around  the  anode,  in  order  to  protect  the  walls  of  the  tube 
from  the  radiation  from  the  hot  anode.  In  discharges  at  high  pressures  this  cylinder 
would  have  little  or  no  effect  on  the  discharge,  but  at  low  pressures,  where  the  free  path 
of  the  electrons  from  the  cathode  becomes  commensurate  with  the  distance  (apparently 
about  2  cm.)  from  the  cathode  to  this  cylinder,  the  effect  is  very  important.  At  very 
low  pressures  these  electrons  charge  up  the  cylinder  to  the  potential  of  the  anode  and 
therefore  practically  destroy  the  potential  gradient  close  to  the  cathode,  where  it  is 
especially  needed  to  remove  the  space  charge.  The  writer  has  often  observed  effects  of 
this  kind  in  connection  with  his  work;  in  fact,  in  very  high  vacuum  the  charging  up  of 
the  glass  sometimes  becomes  very  troublesome.  Thus,  in  some  cases,  after  measuring  a 
thermionic  current  with  240  volts  on  the  anode,  no  current  at  all  will  be  obtained  when 
the  anode  potential  is  changed  to  120  volts.  By  touching  the  bulb  with  the  hand  and 
then  with  the  other  hand  touching  the  positive  terminal  of  the  direct  current  supply 
line,  the  current  instantly  starts  up  again. 

In  Pring  and  Parker's  experiments,  therefore,  as  the  vacuum  improved,  the 
potential  available  for  the  removal  of  the  space  charge  decreased  very  rapidly,  and  this 
effect  is  probably  responsible  for  the  extremely  sir  all  currents  obtained  by  them  in 
some  cases.1 

LILIENFELD'S  EXPERIMENTS2 

Lilienfeld  has  concluded  from  the  results  of  very  careful  and  elaborate  experiments 
in  which  he  took  precautions  to  obtain  a  particularly  high  vacuum,  that  positive  ions 
play  an  essential  role  in  conduction  of  electricity,  even  through  the  highest  vacuum. 

1  Ann.  Phys..  32,  673  (1910). 

a  In  still  more  recent  work  Print?  (Proc.  Roy.  Soc..  89,  344,  1913)  again  finds  the  electron  emission  from  carbon  to  br 
due  entirely  to  secondary  effects.     Undoubtedly  the  lan?e  currents  that  have  often  been  obtained  are  due  to  these  cause 
but  some  measurements  we  have  made  show  clearly  the  existence  01  a  true  e'ectron  emission.     (Sec  Apnendix. 

34 


He  finds  that  beyond  a  certain  point  these  effects  are  entirely  independent  of  the  degree 
of  vacuum.  He  finds  also  that  the  potential  gradient  is  uniform  in  the  space  betweea 
the  electrodes  and  that  the  current  varies  almost  exactly  proportional  to  the  square  of 
the  potential  gradient.  He  shows  from  these  results  that  there  is  no  space  charge  (ex- 
cept at  extremely  low  currents) ,  but  that  there  must  be  equal  numbers  of  positive  and 
negative  ions  in  every  unit  of  volume. 

Lilienfeld  used  a  Wehnelt  cathode  as  a  source  of  electrons,  and  anodes  of  platinum 
foil,  which  were  kept  cold  by  liquid  air  while  the  discharge  passed. 

These  results  are  so  radically  different  from  those  that  have  been  described  in  the 
present  paper  that  they  call  for  comment.  The  effects  observed  by  Lilienfeld  are  cer- 
tainly real,  and  prove  that  the  kind  of  discharge  that  he  was  studying  is  totally  distinct 
from  the  pure  electron  currents  that  have  been  obtained  with  hot  tungsten  cathodes. 

The  essential  difference  in  the  conditions  is  undoubtedly  the  use  of  the  Wehnelt 
cathode. 

From  some  experiments  made  in  this  laboratory  with  Wehnelt  cathodes,  and  judg- 
ing from  the  experiments  of  Child,1  the  writer  is  strongly  of  the  opinion  that  the  Wehnelt 
cathode  is  not  a  primary  source  of  electrons  at  all,  but  is  simply  a  cathode  which  is 
particularly  sensitive  to  bombardment  by  positive  ions  and  under  the  influence  of  such 
bombardment  emits  electrons  copiously. 

In  Lilienfeld's  experiments,  with  the  long  and  crooked  path  between  anode  and 
cathode,  the  space  charge  would  have  prevented  a  pure  electron  discharge  from  taking 
place.  The  Wehnelt  cathode,  however,  probably  liberates  quantities  of  gas  sufficient 
to  furnish  the  positive  ions  necessary.  It  is  also  possible  that  Lilienfeld  obtained  a 
steady  evolution  of  sufficient  gas  by  the  electron  bombardment  of  the  anodes  which 
had  never  been  properly  freed  from  gas. 

APPENDIX2 

Since  the  foregoing  paper  was  written,  a  large  amount  of  data  on  thermionic  cur- 
rents from  various  metals  has  been  obtained  under  conditions  of  still  better  vacuum. 
The  measurements  thus  far  made  are  of  a  preliminary  nature,  but  seem  to  indicate 
that  the  normal  thermionic  current  from  tungsten  is  even  larger  than  that  previously 
given.  Much  more  accurate  determinations  of  the  thermionic  currents  from  tungsten, 
tantalum,  molybdenum,  platinum  and  carbon  are  in  progress,  and  will  be  published. 
The  results  thus  far  show  that  with  all  these  substances,  the  effect  of  residual 
gases  is  always  to  decrease  the  thermionic  current.  Often  the  current  obtained  after 
the  best  vacuum  conditions  have  been  attained  is  100-1,000  times  as  great  as  that  ob- 
served when  only  the  usual  means  of  obtaining  so-called  high  vacuum  are  employed. 

The  following  preliminary  results,  giving  the  observed  thermionic  currents  in  milli- 
amperes  per  sq.  cm.  from  various  filaments  at  2000  deg.  K.,  may  be  of  interest,  but 
should  be  considered  merely  as  lower  limits  to  the  normal  electron  emission  in  a  perfect 
vacuum : 

Thermionic  Current 

at  2(K)J  deg.  K.  Richardson's 

Metal.  Miliiamps.  per  Sq.  Cm.  Constant  b 

Tungsten 3  55,000 

Tantalum 7  50,000 

Molybdenum 1.3  50,000 

Platinum 0.6  80,000 

Carbon 1.0  32,000 


»  Phys.  Rev.,  32,  492  (1911). 
Added  'luring  correction  of  proof. 


3.7 


The  values  of  b  given  in  the  last  column  are  probably  all  slightly  too  high.  With 
platinum  it  is  extremely  difficult  to  get  concordant  results,  probably  because  the  sur- 
face film  is  fairly -stable,  even  close  to  the  melting-point  of  the  metal.  Fredenhagen1 
has  recently  given  reasons  for  concluding  that  none  of  the  measurements  thus  far  made 
of  thermionic  currents  from  platinum  have  really  given  anything  more  than  secondary 
effects.  He  suggests  that  the  presence  of  an  oxide  film  may  seriously  affect  the  results. 
Our  experience  has  fully  confirmed  his  conclusions. 

SUMMARY 

It  is  shown  both  theoretically  and  experimentally  that  the  mutual  repulsion  of  elec- 
trons (space  charge)  in  a  space  devoid  of  positive  ions,  limits  the  current  that  flows 
from  a  hot  cathode  to  a  cold  anode.  For  parallel  plane  electrodes  of  infinite  extent, 
separated  by  the  distance  x,  and  with  a  potential  difference  V  between  them,  the 
maximum  current  (per  unit  area)  that  can  flow  if  no  positive  ions  are  present  is 

V~2 


1  = 


V- 

v  * 


9?r  V  m   x2 

For  the  analogous  case  of  an  infinitely  long,  hot  wire,  placed  concentrically  within 
a  cylindrical  anode,  of  radius  r,  the  maximum  current  per  unit  length  is 

>  =  2\/2~  j~e_y* 
9    Vwr/3"2' 

where  j8  varies  from  0  to  1,  according  to  the  diameter  of  the  wire,  but  for  all  wires  less 
than  1/20  the  diameter  of  the  anode,  /3  is  a  quantity  extremely  close  to  unity. 

2.  In  the  presence  of  gas  at  pressures  above  0.001  mm.,  and  at  voltages  above  40 
volts,  there  is  usually  sufficient  production  of  positive  ions  to  reduce  greatly  the  space 
charge  and  thus  allow  more  current  to  flow  than  indicated  by  the  above  equations. 

3.  It  is  shown,  contrary  to  the  ordinary  opinion,  that  the  general  effect  of  very  low 
pressures  of  gas  is  to  reduce  greatly  the  electron  emission  from  an  incandescent  metal. 

4.  This  effect  is  especially  marked  at  low  temperatures.    In  most  cases  it  probably 
disappears  at  very  high  temperatures. 

5.  The  constant  b  of  Richardson's  equation 

_b 
i  =  aVTe  T 

is  always  increased,  in  the  case  of  tungsten,  by  the  introduction  of  oxygen,  nitrogen, 
water  vapor,  carbon  monoxide  or  dioxide.  Argon,  however,  has  no  effect  on  either  con- 
stant. 

6.  The  normal  thermionic  current  from  tungsten  in  a  "perfect"  vacuum  follows 
Richardson's  equation  accurately.    The  constants  are  approximately: 

a  =  34X106  amps,  per  sq.  cm., 
6  =  55,500. 

7.  Preliminary  data  are  given  for  the  electron  emission  from  tantalum,  molyb- 
denum, platinum  and  carbon.     With  these  substances  also  the  effect  of  gases  is  to 
decrease  greatly  the  electron  emission. 

8.  The  effect  of  nitrogen  in   decreasing  the  thermionic  current  from  tungsten  de- 
pends on  the  voltage  of  the  anode.     In  many  cases  less  current  is  obtained  with  240 
volts  than  with  120  volts.    With  oxygen,  the  effect  seems  independent  of  the  anode 
voltage. 

i  Leipziger  Berichtc,  math.  phys.  Kl.,  65.  42,  1913. 

36 


9.  The  following  theory  seems  to  account  for  most  of  the  observed  phenomena  and 
is  apparently  not  inconsistent  with  any : 

The  effect  of  gases  in  changing  the  saturation  current  is  due  to  the  formation  of 
unstable  compounds  on  the  surface  of  the  wire.  In  the  cases  observed  the  presence  of 
the  compound  decreases  the  electron  emission.  It  is  possible,  however,  that  in  some 
cases  it  might  cause  an  increase.  The  extent  to  which  the  surface  is  covered  by  the 
compound  depends  on  the  rate  of  formation  of  the  compound  and  on  its  rate  of  re- 
moval from  the  surface.  The  compound  may  be  formed  on  the  surface  directly  by  re- 
action with  the  gas  (for  example,  oxygen),  or  by  reacting  principally  with  positive  ions 
which  strike  the  surface  (nitrogen) .  The  compound  may  be  removed  from  the  surface 
by  decomposition,  evaporation,  or  cathode  sputtering  (i.  e.,  being  driven  off  by  bom- 
bardment of  positive  ions). 

10.  The  experimental  conditions  which  should  be  met,  in  order  to  study  most  easily 
the  thermionic  currents  in  high  vacuum,  are  discussed.      It  is  pointed  out  that  failure 
to  observe  these  conditions  is  probably  the  cause  of  other  investigators  having  found 
that  the  thermionic  currents  tend  to  decrease  with  increasing  purity  of  the  cathode 
and  progressive  improvement  of  the  vacuum. 

11.  It  is  concluded  that  with  proper  precautions  the  emission  of  electrons  from  an 
incandescent  solid  in  a  very  high  vacuum  (pressures  below  0.10-6  mm.)  is  an  important 
specific  property  of  the  substance  and  is  not  due  to  secondary  causes. 

In  conclusion,  the  writer  wishes  to  express  his  appreciation  of  the  valuable  assistance 
of  Mr.  S.  P.  Sweetser,  and  Mr.  William  Rogers  who  have  carried  out  most  of  the  ex- 
perimental part  of  this  investigation. 

RESEARCH  LABORATORY, 

GENERAL  ELECTRIC  COMPANY, 
SCHENECTADY,  X.  Y. 


37 


A  POWERFUL  ROENTGEN  RAY  TUBE  WITH  A  PURE 
ELECTRON  DISCHARGE* 

By  W.  D.  COOLIDGE 

RESEARCH   LABORATORY  OF  THE  GENERAL  ELECTRIC  COMPANY,  SCHEXECTADY,    N.   Y. 

§  1.     INTRODUCTION 

In  an  earlier  publication  attention  has  been  called1  to  the  use  of  wrought  tungsten 
for  the  anticathode,  or  target,  of  a  Roentgen  tube  of  the  ordinary  type.  In  the  develop- 
ment of  this  target  many  different  designs  were  made  and  mounted  in  tubes,  and  these 
tubes  were  operated  on  what  was  then  the  most  powerful  Roentgen  apparatus  on  the 
market,  a  10-kw.  transformer  coupled  to  a  mechanical  rectifying  device.  The  opera- 
tion of  tubes  in  this  manner,  to  see  how  much  energy  it  took  to  ruin  the  target,  gave 
perhaps  an  unusual  viewpoint.  When,  as  a  result  of  these  experiments,  a  satisfactory 
form  of  target  had  been  developed,  the  writer  became  interested  in  studying  the 
remaining  limitations  in  the  tube.  Some  of  these  limitations  are  the  following: 

1.  With  low  discharge  currents  the  vacuum  gradually  improves,  with  a  con- 
sequent increase  in  the  penetrating  power  of  the  rays  produced. 

2.  With  high  discharge  currents  there  are  very  rapid  vacuum  changes,  sometimes 
in  one  direction  and  sometimes  in  the  other. 

3.  If  a  heavy  discharge  current  is  continued  for  more  than  a  few  seconds  the  target 
is  heated  to  redness  and  then  gives  off  so  much  gas  that  the  tube  may  have  to  be 
re  xhausted. 

4.  If  the  temperature  of  the  standard  copper-backed  target  is  allowed  to  get  up 
to  bright  redness,  a  rapid  deposition  of  metallic  copper  begins  to  take  place  on  the  bulb, 
continuing  for  some  time  after  the  cutting  off  of  the  current,  owing  to  the  very  slow 
rate  of  cooling  from  such  temperatures  in  the  evacuated  space. 

o.  Of  the  tubes  tested,  very  many  have  failed  from  cracking  of  the  glass,  and 
this,  with  one  exception,  always  at  the  same  point;  that  is,  in  the  zone  around  the 
cathode.  In  many  cases  there  has  first  been  chipping-out  of  the  glass  from -the  inner 
surface  of  the  tube  at  this  point. 

0.  The  focal  spot  on  the  target  in  many  tubes  wanders  about  very  rapidly.'-  In 
many  cases  where  it  does  not  show  a  tendency  to  wander,  it  will  be  found  after  a  heavy 
discharge  to  have  permanently  changed  its  location. 

7.  While  it  is  relatively  easy  to  lower  the  tube  resistance  by  means  of  the  various 
gas  regulators,  it  is  a  relatively  slow  matter  to  raise  it  much. 

5.  With  very  heavy  discharges,  the  central  portion  of  the  usual  massive  alum- 
inum  cathode  melts,  and  the  molten  globules  so  formed  are  shot  right  across  the 
tube,  flattening  themselves  out  on  the  glass  and  sticking  to  it.    When  the  melted 
area  is  small,  no  harm  is  done  except  that  the  curvature  of  the  cathode  at  this  point 
may  be  changed,  and  the  focal  spot  may,  in  consequence,  be  moved. 

*  Copyright,  1913.  by  the  American  Physical  Society. 

From  the  Phys.  Rev.,  Vol.  II,  409-430  (1913). 
1  Coolidge,  Trans.  Am.  Inst.  E.  E.,  June,  1912,  pp.  870-872. 
:  In  radiographic  work,  movement  of  the  focal  spot  during  an  exposure  is  of  course  detrimental  to  good  definition. 

39 


9.     No  two  tubes  are  exactly  alike  in  their  electrical  characteristics. 

10.  The  characteristics  are  in  general  far  from  ideal,  in  that  the  penetrating  power 
of  the  Roentgen  rays  produced  changes  with  the  magnitude  of  the  discharge  current. 

It  was  found  that  limitations  3  and  4  could  be  removed  by  the  use  of  a  massive 
all-tungsten  (in  place  of  the  usual  copper-backed)  target.  Such  a  target  can  be  run 
continuously  at  intense  white  heat.  Aside  from  eliminating  the  troubles  incident  to 
the  use  of  copper,  the  all-tungsten  target  does  not  change  the  general  characteristics 
of  the  tube. 

An  attempt  was  made  to  remove  limitation  8,  imposed  by  the  low  melting  point 
of  aluminum,  by  substituting  for  it  a  tungsten  cathode  of  the  same  dimensions.  Tubes 
made  up  in  this  way  showed  a  behavior  entirely  different  from  that  of  the  ordinary 
tube.  They  would  have  been  absolutely  hopeless  from  the  standpoint  of  a  practical 
radiographer.  They  were  like  the  ordinary  Roentgen  tube  which  is  in  the  condition 
which  the  radiographer  describes  as  "cranky."  Upon  passing  a  discharge  current  of 
any  magnitude  through  the  tube,  the  resistance  would  quickly  rise  to  a  point  where, 
even  with  an  impressed  potential  difference  of  100,000  volts,  no  further  discharge 
would  pass.  The  tube  could  be  restored  to  its  original  condition  by  the  liberation  of 
gas  from  the  vacuum  regulator.  The  phenomena  would  repeat  themselves  as  often  as 
one  cared  to  make  the  experiment.  Continuous  operation  for  even  a  few  seconds 
seemed  out  of  the  question.  It  finally  developed,  however,  that  with  the  adoption 
of  the  following  expedient,  the  situation  changed.  Gas  was  admitted  from  the  regulator 
and  a  discharge  passed  through  the  tube.  As  soon  as  the  resistance  had  risen,  more 
gas  was  admitted  and  the  tube  was  again  excited.  These  operations  were  repeated  as 
rapidly  as  possible.  With  each  excitation  of  the  tube,  the  cathode  became  hotter. 
There  was  evidently  the  same  focusing  of  the  positive  ions  bombarding  the  cathode  as 
there  was  of  the  electrons  at  the  anticathode,  for  there  was  a  similar  localization  of 
heat  at  the  two  electrodes.  A  little  conical  depression  which  formed  at  the  center  of 
the  cathode  showed  that,  with  the  vacuum  range  employed,  the  positive  ion  bombard- 
ment was,  at  least  mainly,  confined  to  an  area  only  about  2  mm.  in  diameter.  As  soon 
as  the  cathode  had  become  heated  to  bright  incandescence,1  the  behavior  of  the  tube 
changed,  and  it  could  then  be  operated  continuously  for  at  least  several  minutes. 
Upon  interrupting  the  discharge  for  a  short  time,  and  so  allowing  the  cathode  to  cool, 
the  "cranky"  condition  returned,  and  the  tube  could  again  be  operated  continuously 
only  after  repeating  the  procedure  outlined  above. 

Tubes  like  the  above  with  tungsten  cathodes  showed,  upon  operation,  a  rapid 
blackening  of  the  bulb.  The  deposit  proved  to  be  metallic  tungsten.  The  conical 
depression  which  invariably  formed  at  the  center  of  the  cathode  seemed  to  indicate 
that  this  was  the  source  of  the  deposit,  the  disintegration  of  the  metal  at  this  point 
being  doubtless  due  to  the  mechanical  action  of  the  positive  ions  which  bombard  it. 

The  extreme  instability  of  vacuum  attendant  upon  the  use  of  a  tungsten  cathode 
in  what  was  otherwise  a  standard  Roentgen  tube  called  attention  very  forcibly  to  the 
part  which  is  played  by  gases  in  the  ordinary  aluminum  cathode.  (The  tungsten 
cathodes  used  must  have  been,  from  their  method  of  manufacture,  relatively  very  free 
from  gas.) 

A  consideration  of  the  above-mentioned  limitations  showed  that  they  were  for 
the  most  part  incident  to  the  use  of  gas  and  that  they  could  therefore  be  made  to  dis- 


appear  if  a  tube  could  be  operated  with  a  very  much  higher  vacuum.  In  this  case  the 
electrons  would  have  to  be  supplied  in  some  other  way  than  by  bombardment  of  the 
cathode  by  positive  ions. 

Richardson1  and  others  had  shown  that  electrons  might  be  produced  by  simply 
heating  the  cathode.  But  the  values  of  the  thermionic  currents  obtained  by  different 
observers  had  varied  between  wide  limits,  so  much  so  as  to  suggest  that  a  Roentgen 
tube  based  upon  this  principle  might  be  as  unstable  in  resistance  as  is  the  standard 
tube.  Moreover,  the  fact  that  the  substances  usually  worked  with,  platinum  and 
carbon,  are  so  difficult  to  completely  free  from  gas,  suggested  strongly  that  with  the 
cleaner  conditions  (greater  freedom  from  gas)  that  could  be  realized  by  the  use  of 
tungsten,  the  thermionic  currents  might  cease  altogether.2  Some  experiments  of  Dr. 
Irving  Langmuir,3  however,  on  the  thermionic  currents  between  two  tungsten  fila- 
ments in  a  highly  evacuated  space,  were  very  reassuring.  According  to  his  observa- 
tions, after  a  certain  high  degree  of  exhaustion  had  been  reached,  the  thermionic 
currents  increased,  up  to  a  certain  limiting  value,  as  the  tube  became  freer  and  freer 
from  gas. 

The  idea  of  using  a  hot  cathode  in  a  Roentgen  tube  was  not  new,  but,  so  far  as  the 
writer  could  learn,  the  principle  had  never  been  successfully  applied  in  a  vacuum  good 
enough  so  that  positive  ions  did  not  play  an  essential  r61e. 

Wehnelt  and  Trenkle4  had  used  a  hot  lime  cathode  for  the  production  of  very  soft 
Roentgen  rays,  working  with  voltages  from  400  to  1000.  Wehnelt,  in  another  article, 
describes  the  use  of  his  lime  cathode  in  a  Braun  tube,  and  here  he  says  that  it  is  not 
advisable  to  employ  more  than  1000  volts,  as  otherwise  cathode  rays  come  off  from 
that  part  of  the  platinum  which  is  bare,  giving  bad  disintegration. 

The  Roentgen  rays  produced  by  voltages  as  low  as  1000  are,  of  course,  too  "soft" 
for  the  ordinary  applications. 

Lilienfeld  and  Rosenthal5  had  described  a  Roentgen  tube  whose  penetrating  power 
is,  they  say,  independent  of  vacuum.  Their  main  aluminum  cathode  and  their  platinum 
anticathode  are  shaped  and  located  like  the  electrodes  in  the  ordinary  Roentgen  tube. 
Besides  these  they  have  an  anode  and  an  auxiliary  hot  cathode.  Current  from  a  low 
voltage  source  passes  from  the  hot  cathode  to  the  anode  and  this  current  furnishes  the 
positive  ions  which  by  their  bombardment  of  the  main  cathode  liberate  electrons  from 
it.  Their  tube  is  dependent  for  its  operation  on  the  presence  of  positive  ions,  for  with- 
out these  there  is  no  means  provided  for  getting  electrons  out  from  the  main,  alu- 
minum, cathode.  Lilienfeld  concludes  from  his  extended  experiments  in  tube  exhaustion 
that  the  complete  removal  of  all  gas  from  tube  and  electrodes  would  not  do  away  with 
positive  ions.  There  would  according  to  this  view  be  no  such  thing  as  a  pure  electron 
discharge.  Lilienfeld's  work  in  exhausting  the  gas  from  the  tube  itself  and  from  the 
glass  seems  to  have  been  excellent,  but  according  to  the  experience  of  the  writer,  his 
electrodes  were  not  sufficiently  freed  from  gas  to  justify  the  conclusions  drawn.  Work- 
ing even  with  tungsten  electrodes  in  a  tube  so  designed  that  the  electrodes  could  be 
heated  in  place  to  very  high  temperatures,  the  writer  has  had  the  positive  ion  effects 
persist  for  hours,  disappearing  completely  however  as  the  electrodes  become  sufficiently 
freed  from  gas. 

1  Proc.  Camb.  Phil.  Soc.,  XI,  286  (1902);  Proc.  Roy.  Soc.,  LXXI,  pp.  415-418  (1903). 

2  See  Goldstein,  Ann.  der  Physik,  24,  p.  91,  1885.     He  finds  that  platinum  must  be  heated  almost  to  its  melting  point 
to  give  an  appreciable  thermionic  current.    Also  see  H.  A.  Wilson,  Proc.,  Roy.  Soc.,  72,  pp.  272-276  (1903).     He  concludes 
with  the  following  statement:    "It  is  probable  that  a  pure  platinum  wire  heated  in  a  perfect  vacuum  would  not  discharge 
any  electricity  at  all,  either  positive  or  negative,  to  an  extent  appreciable  on  a  galvanometer." 

3  This  work  is  just  being  published. 

«  A.  Wehnelt  and  W.  Trenkle,  Sitzungsber.  d.  Phys- Medic.  Soc.  in  Erlangen,  37,  312-315  (1905). 

5  J.  E.  Lilienfeld  and  W.  J.  Rosenthal.  Fortschritte  auf  dem  Gebiete  der  Rontgenstrahlen,  18,  256-263  (1912). 

41 


The  work  of  Dr.  Langmuir  had  shown  that  a  hot  tungsten  cathode  in  a  very  high 
vacuum  could  be  made  to  yield  continuously  a  supply  of  electrons  at  a  rate  determined 
by  the  temperature. 

Further  work  showed  that  very  high  voltages,  up  to  at  least  100, 000,  in  no  wise 
affect  this  rate  of  emission.  For  application  to  the  fields  of  radiography  and  fluoro- 
scopy,  it  was  necessary  to  develop  a  satisfactory  method  of  focusing.  And,  finally, 
the  lai  ge  amounts  of  energy  transformed  into  heat  in  a  Roentgen  tube  render  imperative 
the  use  of  a  very  heavy  target,  and  this  made  it  necessary  to  develop  methods  for 
sufficiently  freeing  frcm  gas  large  masses  of  metal. 

The  result  of  efforts  in  this  direction  has  been  entirely  successful,  and  tubes  have 
been  made,  based  upon  this  principle,  which  are  free  from  all  of  the  above-mentioned 
limitations.  This  is  of  particular  physical  interest  because  it  brings  all  of  the  peculiar- 
ities of  the  Roentgen  ray  tube  into  accord  with  the  modern  conception  of  electronic 
conduction  and  gas  molecule  decomposition.  In  the  following,  it  will  be  sufficient  to 
describe  one  type  (a  focusing  tube)  and  its  characteristics,  leaving  for  later  papers 
the  description  of  other  types. 

§  2.     GENERAL  DESCRIPTION  OF  THE  NEW  TYPE  OF  TUBE 

The  structural  features  of  the  new  tube  which  differ  from  those  of  the  ordinary 
type  are  the  following: 

The  pressure,  instead  of  being,  as  in  the  ordinary  tube,  a  few  microns,  is  as  low  as  it 
has  been  possible  to  make  it,  that  is,  not  more  than  a  few  hundredths  of  a  micron. 

The  cathode  consists  of  a  body  which  can  be  electrically  heated  (such  as  a  tungsten 
or  tantalum  filament)  and,  suitably  located  with  reference  to  this  portion,  an  electrically 
conducting  ring  or  cylinder,  consisting  preferably  of  molybdenum  or  tungsten  or  other 
refractory  metal.  The  ring  or  cylinder  is  connected  either  to  the  heated  portion  of  the 
cathode,  or  to  an  external  source  of  current  by  means  of  which  its  potential  may  be 
brought  to  any  desired  value  with  respect  to  the  heated  portion.  The  heated  portion 
of  the  cathode  serves  as  the  source  of  electrons,  while  the  ring  or  cylinder  assists  in  so 
shaping  the  electrical  field  in  the  neighborhood  of  the  cathode  that  the  desired  degree 
of  focusing  of  the  cathode-ray  stream  upon  the  target  shall  result. 

The  anticathode,  or  target,  functions  at  the  same  time  as  anode. 

The  operation  is  satisfactory  only  when  the  vacuum  is  exceedingly  high,  so  high 
that  the  ordinary  tube  would  carry  no  current  even  on  100,000  volts. 

§  3.     THEORY  OF  OPERATION 

As  will  be  seen  from  the  characteristics  of  the  tube,  in  §  5,  it  gives,  in  operation,  no 
evidence  of  positive  ions.  This  makes  the  theory  of  its  operation  exceedingly  simple. 

The  dischaige  appears  to  be  purely  thermionic  in  character. 

The  rate  of  emission  of  electrons  from  the  filament  appears  to  be  in  accord  with 
Richardson's  Law,  which  says  that  the  maximum  thermionic  current,  \\hich  can  be 

drawn  from  a  hot  filament  is 

/„.     i> 
i  =  aV  1  e  Y" 

where  T  is  the  absolute  temperature,  e  is  the  base  of  the  natural  system  of  logarithms, 
and  a  and  b  are  constants. 

In  the  particular  tube  described  in  detail  in  this  paper,  over  the  range  of  tempera- 
tures and  voltages  included  in  the  data  of  Table  I,  this  simple  law  accounts  perfectly 
for  the  conducthity  of  the  tube.  With  still  higher  temperatures,  however,  the  dis- 
charge currents  would  be  found  to  increase  at  a  much  slower  rate  than  that  required 

42 


by  the  above  law.  And  the  same  applies,  even  in  the  temperature  range  of  Table  I,  to 
a  different  tube  design  in  which  the  distance  between  cathode  and  anode  is  greater. 
In  these  cases  the  failure  to  follow  Richardson's  Law  at  the  higher  temperatures  has 
been  accounted  for  by  Dr.  Largmuir1  by  the  spacial  density  of  negative  electricity  in 
the  neighborhood  of  the  cathode. 

§  4.     DETAILED  DESCRIPTION  OF  TUBE  No.  147 

This  description  relates  to  tube  No.  147,  which  was  used  in  getting  the  data  for 
the  following  tables.  Fig.  1  shows  a  complete  assembly,  while  Fig.  2  shows  an  enlarged 
detail  of  the  cathode  and  of  the  front  end  of  the  target 


1    V        ','         '.'    1 


Fig.  1 

The  Cathode 

In  the  diagrams,  25  is  a  tungsten  filament  in  the  shape  of  a  flat,  closely  wound 
spiral.  It  consists  of  a  wire  0.21(5  mm.  in  diameter  and  33.4  mm.  long  with  5j/£  con- 
volutions, the  outermost  of  which  has  a  diameter  of  3.5  mm.  It  is  electrically  welded 
to  the  ends  of  two  heavy  molybdenum  wires  14  and  15,  to  the  other  extremities  of 
which  are  welded  the  two  copper  wires  16  and  17.  These  in  turn  are  welded  to  the 
platinum  wires  18  and  19.  The  molybdenum  wires  are  sealed  directly  into  a  piece  of 
special  glass,  12,  which  has  essentially  the  same  tempera- 
ture coefficient  of  expansion  as  molybdenum.  This  first 
seal  is  simply  to  insure  a  rigid  support  for  the  hot  filament, 
the  outer  seal  being  the  one  relied  upon  for  vacuum  tight- 
ness. The  outer  end,  13,  of  the  support  tube  is  of  German 
glass  like  the  bulb  itself,  and  it  is,  therefore,  necessary  to 
interpose  at  5  a  series  of  intermediate  glasses  to  take  care  of 
the  difference  in  expansion  coefficients  between  12  and  13. 
prevents  short-circuiting  of  the  copper  wires,  16  and  17. 

The  filament  is  heated  by  current  from  a  small  storage  battery  which  is,  electrically, 
well  insulated  from  the  ground. 

In  the  circuit  are  placed  an  ammeter  and  an  adjustable  rheostat  and,  by  means  of 
the  latter,  the  filament  current  can  be  regulated,  by  very  fine  steps,  from  3  to  5  amperes. 
Over  this  current  range,  the  potential  drop  through  the  filament  varies  from  1.8  to 
4.6  volts  and  the  filament  temperature  from  1890  to  2540  degrees  absolute. 

The  Focusing  Device 

This  consists  of  a  cylindrical  tube  of  molybdenum,  21.  It  is  (5.3  mm.  inside  diameter 
and  is  mounted  so  as  to  be  concentric  with  the  tungsten  filament,  and  so  that  its  inner 

'L.  c. 


Fig.  2 


The  small  glass  tube,  20, 


4:5 


end  projects  1.0  mm.  beyond  the  plane  of  the  latter.  It  is  supported  by  the  two  stout 
molybdenum  wires,  22  and  23,  which  are  sealed  into  the  end  of  the  glass  tube,  12.  It 
is  metallically  connected  to  one  of  the  filament  leads,  at  24. 

Besides  acting  as  a  focusing  device,  it  also  prevents  any  discharge  from  the  back 
of  the  heated  portion  of  the  cathode. 

The  Anticathode  or  Target 

The  anticathode  or  target,  2,  which  also  serves  as  anode,  consists  of  a  single  piece 
of  wrought  tungsten,  having  at  the  end  facing  the  cathode  a  diameter  of  1.9  cm. 
(Its  weight  is  about  100  gm.)  By  means  of  a  molybdenum  wire,  5,  it  is  firmly  bound 
to  the  molybdenum  support,  6.  This  support  is  made  up  of  a  rectangular  strip  and, 
riveted  to  this,  three  split  rings,  11,  11,  11,  all  of  molybdenum.  The  split  rings  fit 
snugly  in  the  glass  anode  arm,  7.  They  serve  the  double  purpose  of  properly  support- 
ing the  anode  and  of  conducting  heat  away  from  the  rectangular  strip  and  so  preventing 
too  much  heat  flow  to  the  seal  of  the  lead-in- wire,  9. 


Fig.  3 

The  Bulb 
This  is  of  German  glass  and  about  18  cm.  in  diameter. 

The  Exhaust 

This  is  as  thorough  as  possible. 

For  the  earlier  tubes,  mercury  pumps  were  used,  with  a  liquid-air  trap  between 
tube  and  pump  to  eliminate  mercury  vapor.  The  whole  tube,  while  connected  to  the 
pump,  was  in  an  oven  and  was  heated  at  intervals  to  470  deg.  C.  Between  heating 
operations  the  tube  was  operated  with  as  heavy  discharge  currents  as  the  condition 
of  its  vacuum  would  permit.  For  hours  the  tube  would  show  the  characteristics 
of  an  ordinary  Roentgen  tube,  and  in  many  cases  a  several  days'  application  of  the 
above  treatment  was  required  to  entirely  eliminate  these  characteristics  and  to  realize 
an  essentially  pure  electron  discharge. 

The  exhaust  time  has  been  greatly  reduced  in  two  ways.  The  massive  tungsten 
anode  is  given  a  preliminary  firing  to  a  very  high  temperature  in  a  tungsten-tube 

44 


vacuum  furnace.1  The  molybdenum  support  is  also  fired,  to  a  somewhat  lower  tem- 
perature, in  the  same  manner.  In  the  second  place,  a  Gaede  molecular  pump  has  been 
substituted  for  the  mercury  pumps  and,  at  the  same  time,  a  very  large  and  short  con- 
nection has  been  adopted  between  tube  and  pump. 

In  the  later  stages  of  the  exhaust  a  very  heavy  discharge  current  is  maintained 
continuously  on  the  tube  for  perhaps  an  hour,  the  temperature  of  the  bulb  being  kept 
from  rising  too  high  by  the  use  of  a  fan. 

The  pressure  in  the  finished  tube  is  very  low,  certainly  not  more  than  a  few  hun- 
dredths  of  a  micron  and  probably  much  less  than  this. 

Connections  and  Method  of  Operating 

The  tube  was  connected  as  shown  in  the  diagram,  Fig.  3,  in  which,  T  is  the  tube; 
B  is  a  small  storage  battery;  A  is  an  ammeter;  R  is  an  adjustable  rheostat  which  can  be 
controlled  from  behind  the  lead  screen  which  shields  the  operator  from  the  Roentgen 
rays;  S  is  an  adjustable  spark  gap  with  pointed  electrodes,  which  can  also  be  operated 
from  behind  the  lead  screen;  and  M  is  a  milliampere-meter  which  can  be  read  from 
behind  the  screen. 

As  the  high  potential  is  connected  to  the  battery  circuit  it  is  necessary  that  the 
latter  shall  be  thoroughly  insulated  from  the  ground. 

As  a  high  potential  source,  a  10-kw.  Snook  machine,  made  by  the  Roentgen  Appa- 
ratus Co.,  was  used.  This  consists  of  a  rotary  converter  driven  from  the  direct  current 
end  and  delivering  alternating  current  at  150  volts  and  60  cycles  per  second  to  a  closed 
magnetic  circuit  step-up  transformer  with  oil  insulation.  From  the  secondary  of  this 
transformer  the  high  voltage  current  is  passed  through  a  mechanical  rectifying  switch 
(which  is  direct-connected  to  the  shaft  of  the  rotary)  and  the  milliampere-meter,  M,  to 
the  tube.  The  output  of  the  transformer  is  controlled  by  a  variable  resistance  in  the 
primary. 

Throughout  these  experiments  a  fan  was  kept  blowing  on  the  tube.  Without  this 
fan,  the  gas  pressures  in  the  tube  would  be  slightly  higher,  and  the  discharge  currents 
would  be  in  consequence  slightly  lower. 

§  5.     CHARACTERISTICS 
A.     No  Discharge  Current  Unless  Filament  is  Heated 

Unless  the  filament  is  heated,  the  tube  shows  no  conductivity  in  either  direction, 
even  with  voltages  as  high  as  100,000. 

B.     Tube  Allows  Current  to  Pass  in  Only  One  Direction 

The  tube  suppresses  any  current  in  the  direction  which  does  not  make  the  hot 
filament  cathode.  It  is  therefore  capable  of  rectifying  its  own  current  when  supplied 
from  an  alternating  source. 

In  the  case  of  a  focusing  tube,  however,  the  use  of  alternating  current  will  very 
considerably  lower  the  maximum  allowable  energy  input.  For  as  soon  as  the  target 
becomes  heated  at  the  focal  spot  to  a  temperature  approximating  that  of  the  filament, 
the  tube  will  cease  to  completely  rectify,  and,  as  the  temperature  of  the  focal  spot  rises, 
will  allow  more  and  more  current  to  pass  in  the  wrong  direction.  This,  to  be  sure,  will 
not  cause  either  a  harmful  vacuum  change  or  a  metallic  deposit  on  the  bulb,  as  it 
would  in  the  case  of  the  ordinary  tube,  but  it  will  give  rise  to  needless  heating  of  the 
bulb  where  it  is  bombarded  by  the  cathode  rays  from  the  target,  and  to  disturbing 
Roentgen  rays  emanating  from  the  glass  at  this  point.  In  the  case  of  a  tube  which  does 

1  A  description  of  this  furnace  will  be  published  in  the  near  future.  The  heating  element  consists  of  a  tungsten  tube 
2.5  cm.  inside  diameter  and  30  cm.  long.  This  is  fastened  in  an  upright  position  and,  by  means  of  suitable  terminals,  is 
connected  to  a  100-kw.  transformer.  The  heating  element  is  placed  in  a  water-cooled  metal  cylinder  and  the  space  within 
connected  to  a  pump  which  maintains,  with  the  furnace  at  its  highest  temperature,  a  vacuum  of  a  few  microns. 

45 


not  focus,  but  in  which  the  cathode  rays  bombard  the  entire  surface  of  the  anode,  the 
allowable  energy  input  which  the  tube  will  completely  rectify  can  be  increased  to 
any  desired  amount  by  simply  increasing  the  surface  of  the  anode. 

C.     Discharge  Current  Determined  Primarily  by  Filament  Temperature 

With  a  given  design,  the  amount  of  discharge  current  which  can  be  passed  through 
the  tube  is  determined  primarily  by  the  temperature  of  the  filament,  and  responds 
instantly  to  changes  in  the  same  in  either  direction. 

The  effect  of  both  temperature  and  voltage  on  the  discharge  current,  in  the  case  of 
tube  No.  147,  illustrated  in  Fig.  1 ,  may  be  seen  by  referring  to  Table  I,  which  gives  the 
data  on  the  finished  tube  after  it  had  been  sealed  off  from  the  pump.  The  focal  spot 
was  3  mm.  in  diameter. 

In  the  table,  Column  I  gives  the  length  in  centimeters  of  the  equivalent  spark  gap. 

Column  II  gives  the  heating  current  (C)  in  the  filament,  expressed  in  amperes. 


1 


30 


20OO 


2/OO 
Fit.  4 


3200 


Column  III  gives  the  filament  temperature  (T),  expressed  in  degrees  absolute, 
corresponding  to  the  values  of  C  in  Column  II.  The  temperature  values  were  obtained 
by  comparison  with  a  previously  calibrated  tungsten  lamp. 

Column  IV  gives  the  discharge  current  (i)  through  the  tube,  in  milliamperes. 

Column  V  gives  the  calculated  values  of  (—  log  vV  T)  to  the  base  10. 

Column  VI  gives  calculated  values  of  (0.434/  TX  106). 

To  obtain  the  data  in  the  table,  the  experimental  procedure  was  as  follows :  The 
filament  current  was  first  set  at  a  predetermined  point.  The  spark  gap  was  next  set, 
also  at  a  predetermined  point.  The  tube  was  then  excited  and  the  voltage  across  the 
tube  terminals  adjusted  (by  varying  the  resistance  in  the  primary  circuit  of  the  trans- 
former) until  sparks  were  occasionally  jumping  across  the  parallel  spark  gap.  The 
discharge  current  value  was  then  read  off  from  the  milliammeter. 

The  filament  current  was  then  raised  to  a  second  predetermined  value.  This 
increased  the  discharge  current  and  lowered  the  potential  across  the  tube  terminals 


The  latter  was  then  raised  to  its  original  value  and  the  new  discharge  current  reading 
was  obtained. 

In  this  way  the  discharge  current  value,  for  a  given  voltage,  was  brought  up  by 
steps  to  the  point  where  the  tube  finally  began  to  show  signs  of  instability.  The  tem- 
perature-current series  was  then  repeated  with  a  different  voltage. 

The  values  of  discharge  current  and  temperature  for  each  voltage  are  plotted  in 
Fig.  4.  The  different  curves  are  seen  to  lie  very  close  together,  showing  that  over  the 
range  of  voltages  employed,  the  magnitude  of  the  discharge  current  is  practically 
independent  of  voltage.  This  shows  that  the  current  in  these  experiments  was  always 
the  saturation  value.1 

TABLE   I 


. 

ii 

Filament 
Current  C 
(Amps.) 

Ill                                                 IV 

V 

VI 

Equivalent 
Spark  Gap 
(Cm.) 

Filament                     Discharge 
Temp.  7"                     Current  / 
(Degs.  Abs.)               (Milliamps.) 

j 

434 
^?X10* 

-L°S/T 

4                           3.40 

2010                          1.7 

1.4212 

216.1 

3.45 

2028                         3.5 

1.1094 

214.1 

3.51 

2049 

6.4 

.4985 

212.0 

3.60 

2077 

11.3 

.6056 

209.1 

3.66 

2088 

15.8 

.4611 

208.0 

3.67 

2104 

24.0 

.2813 

206.4 

3.71 

2116 

27.0 

.2313 

205.3 

3.73 

2121 

40.0 

.0611 

204.8 

6                           3.29 

1976 

1.6 

1.4438 

219.8 

3.43 

2020 

3.3 

1.1341 

215.0 

3.52 

2053 

6.5 

.8433 

211.5 

3.59 

2074 

10.6 

.6331 

209.4 

3.64 

2090 

15.3 

.4753 

207.8 

3.70 

2110                       22.3 

.3138 

205.8 

3.72 

2120 

28.3 

.2113 

204.9 

8 

3.27 

1970 

1.6 

1.4431 

220.4 

3.43 

2020 

2.9 

1.1902                       215.0 

3.53 

2055 

5.7 

.9005                       211.3 

3.59 

2074 

9.7 

.6716 

209.4 

3.64 

2090 

13.8 

.5201 

207.8 

. 

3.71 

2116                       21.8 

.3242 

205.3 

3.73 

2121                       26.2 

.2449 

204.8 

4.07 

2240                       36.2 

.1164 

193.9 

10 

3.09 

1909                         0.6 

1.8411 

227.5 

3.31 

1980                         2.5 

1.2504 

219.4 

3.40 

2010                         4.4 

1.0081 

216.1 

3.50 

2046                         8.2 

.7416 

2,12.3 

3.57 

2070                       12.6 

.5576 

209.8 

3.67 

2104                       20.7 

.3455 

206.4 

3.65 

2096                       21.8 

.3222 

207.2 

3.71 

2116                       27.0 

.2313 

205.2 

4.13 

2259                       35.4 

.1279 

192.3 

12 

3.28 

1973                          1.7 

1.4171                       220.2 

3.44 

2023                         3."4 

1.1215 

214.7 

3.55 

2061                          7.3 

.7937 

210.7 

3.57 

2070                       10.1 

.6537 

209.8 

3.65 

2093                       16.9 

.4325 

207.5 

3.68 

2107                       22.4 

.3116 

206.1 

3.68 

2107                       20.1 

.3586                       206.  1 

1  4                           3.  1  1 

1917                          1.0 

1.6413                       226.6 

3.36 

1998 

2.5 

1.2624                       217.4 

3.52 

2053 

4.5 

1.0030                       211.5 

. 

3.64 

2090 

8.6 

.7255                      207.8 

3.71 

2116                       14.7 

.4954                       205.3 

3.72 

2120                        19.6 

.3708                       204.9 

3.73 

2121                       26.0 

.2482                       204.8 

1  The  two  points  to  the  extreme  right  of  the  curves  correspond  to  an  unstable  condition  of  the  tube, 
disappears  instantly  upon  lowering  the  filament  temperature. 


The  instability 


According  to  Richardson,  the  relation  between  the  saturation  current  flowing  from 
a  hot  filament  and  the  absolute  temperature  of  the  filament  is  expressed  by  the 
equation  •i  =  av/Te  f,  in  which  i  is  the  current,  T  is  the  temperature,  and  a  and  b  are 
constants  of  which  the  first  has  to  do  with  the  concentration  of  electrons  within  the 
hot  body,  and  the  second  represents  the  amount  of  work  required  to  get  the  electron 
through  the  surface  of  the  metal,  e  is  the  base  of  the  Naperian  system  of  logarithms. 

Richardson1  applies  this  equation  to  his  data  by  first  taking  the  logarithm,  to  the 
base  10,  of  both  sides  of  the  equation,  which  gives 

log  *  =  log  a  +  J  log  T- (0.434) 


or 


-log- 


.434 
~T~ 


-log  a, 


which  is  the  equation  of  a  straight  line. 


If  then  the  values  of  Columns  IV  and  V  are  plotted,  they  should  lie  along  straight 
lines,  provided  conduction  in  the  tube,  between  the  voltage  limits  used,  follows  Richard- 
son's Law. 

Reference  to  the  plots,  Figs.  5  and  6,  will  show  that  the  points  are  closely  rep- 
resented by  straight  lines.  By  reading  off  the  tangents  of  the  angles  which  the  lines 
make  with  the  horizontal  axis  we  get  the  following  values  of  the  constant  b: 

Voltage  Corresponding 

to  Spark  Gap  of  Value  of  b 

4  cm 115,000 

6  cm 93,000 

8  cm 94,000 

10  cm 71,000 

12  cm 76,000 

14  cm 60,000 

Average 85,000 


W.  Richardson,  Proc.  Camb.  Phil.  Soc.,  11,  p.  293  (1901). 


48 


11" 


L^u 


FIE 
ra 


^x 


.•4.34- 


..__. 


.?<W  <?/0  2/.T 

Fig.  6 

These  values  of  6  are  interesting  in  that  they  all  fall  within  the  range  of  the  values 
which  are  just  being  published  by  Dr.  Langmuir.  His  values  were  obtained  from  an 
apparatus  in  which  the  electrodes  were  fine  tungsten  filaments  having  a  total  mass 
perhaps  1/100,000  of  that  of  the  target  in  the  above  tube.  His  results  show  the 
enormous  effect  of  gas  on  the  value  of  6,  and  the  conclusion  can,  therefore,  be  drawn 
that  very  large  tungsten  masses  can  be  used  in  a  tube  without,  to  any  appreciable 
extent,  impairing  the  vacuum,  even  though  these  masses  may  be  heated  close  to  their 
melting  point. 

If  the  temperature  of  the  filament  is  low,  only  a  small  number  of  electrons  escape 
from  it  and,  consequently,  only  a  small  discharge  current  (the  saturation  current)  can 
be  sent  through  the  tube.  Increasing  the  impressed  voltage  above  that  needed  for 
this  current  value  causes  no  further  increase  in  current.  It  simply  increases  the  velocity 
of  the  cathode  rays  and  hence  the  penetrating  power  of  the  Roentgen  rays. 

At  higher  filament  temperatures  there  is  a  current-limiting  factor,  other  than  the 
number  of  electrons  emitted  by  the  hot  filament,  which  plays  a  dominating  part.  This 
factor  is  the  spacial  density  of  negative  electricity  in  front  of  the  cathode,  which 
amounts,  in  effect,  to  a  back  electromotive  force.  This  factor  would  play  a  very 
important  role  at  lower  voltages;  but  in  the  case  of  the  Roentgen  ray  tube  the  voltages 
involved  are  so  high  that,  in  case  a  suitable  design  is  used,  its  influence  can  be  entirely 
avoided,  as  is  shown  by  the  data  of  Table  I. 

D.     Penetrating  Power  of  Roentgen  Rays  Determined  by  Voltage  Across  Tube 

Terminals 

The  penetrating  power  of  the  Roentgen  rays  coming  from  the  tube  increases  with 
the  potential  difference  between  tube  terminals. 

With  the  tube  excited  from  a  variable  potential  source,  such  as  the  transformer,  it  did 
not  seem  safe  to  predict  that,  with  the  same  equivalent  spark  gap,  the  rays  would  show, 
photographically,  the  same  penetrating  power  as  those  from  a  standard  tube.  But  upon 
making  the  experiment,  using  a  Benoist  penetrometer,1  it  was  found  that  they  did. 

The  experiment  was  interesting  from  another  point  of  view  in  that  it  showed  how 
readily  the  new  tube  could  be  adapted  to  a  given  set  of  conditions. 

An  exposure  was  made  first  with  a  standard  tube  of  the  ordinary  type,  and  the 
discharge  current  and  equivalent  spark  gap  were  noted.  A  tube  of  the  new  type  was 


M.  L.  Benoist,  C.  R.,  134,  225  (1902). 


49 


then  set  up  in  place  of  the  standard.  It  was  but  the  work  of  a  moment  to  adjust  the 
new  tube  to  the  point  where  it  showed  the  same  discharge  current  and  equivalent  spark 
gap  as  the  standard  tube.  The  radiographs  of  the  penetrometer,  made  with  the  two 
tubes,  showed  the  same  penetration  number. 

E.     Capable  of  Continuous  Operation  Without  Change  of  Cltaracteristics 

That  the  tube  may  be  operated  continuously  without  showing  an  appreciable 
change  in  characteristics  is  shown  by  the  following  experiment  on  Tube  No.  147, 
illustrated  in  Fig.  1 . 

The  filament  current  was  set  at  4.1  amperes.  This  gave  a  discharge  current  of 
25  milliamperes.  The  impressed  voltage  was  then  set  at  a  point  where  the  tube  showed 
a  7-cm.  equivalent  spark  gap. 

The  tube  was  then  run  continuously,  with  no  adjustment  of  any  kind,  for  50 
minutes.  The  readings  of  discharge  current  and  equivalent  spark  gap,  taken  every 
two  minutes,  are  given  in  Table  II. 

F.     Sharpness  of  Focus 

Sharpness  of  focus  is  determined  mainly  by  the  design  of  the  tube.  If  the  filament 
temperature  is  such  that,  with  the  voltage  employed,  the  discharge  current  does  not  repre- 
sent the  saturation- value,  the  size  of  the  focal  spot  will  vary  with  the  impressed  voltage. 
But  in  the  tube  shown  in  Fig.  1 ,  over  the  range  of  voltages  corresponding  to  an  equiva- 
lent spark  gap  ranging  from  4  to  14  cm. .the  focal  spot  does  not  vary  appreciably  in  size. 

The  tube  may  be  made  to  focus  more  sharply  by  increasing  the  distance  between  the 
filament  and  the  front  (end  facing  the  target)  of  the  molybdenum  tube.  This  change 
in  design  will  also  affect  the  temperature-current  characteristics  of  the  tube  in  the  direc- 
tion that  a  higher  filament  temperature  will  be  needed  for  a  given  discharge  current  value. 

Similarly  it  may  be  made  to  focus  less  sharply  by  decreasing  the  distance  between 
the  filament  and  the  front  of  the  focusing  device. 

All  of  the  observations  made  are  consistent  with  the  idea  that  focusing  is  determined 
by  the  shape  of  the  equipotential  surfaces  which  may  be  drawn  in  the  space  between 
cathode  and  anode,  and  that  the  surfaces  close  to  the  cathode  have  the  strongly  pre- 
ponderating influence. 

TABLE  II 


-p. 


Discharge  Current 
(Milliamps.) 


Equivalent  Spark  Gap 
(Cm.) 


11:48  a.m.                    25                       7.0 

:50 

25 

7.0 

:52 

25 

6.9 

:54 

25 

6.5 

:56 

25 

6.5 

12:00  p.m. 

25 

6.7 

:02 

25 

6.9 

:04 

25 

6.5 

:06 

24 

6.4 

:()8 

24                     6.5 

:10 

24 

6.5 

:12 

23 

6.6 

:14 

25 

7.0 

•16 

25 

6.8 

:18 

24 

6.8 

•20 

25 

6.9 

:23V4 

23 

6.7 

!26 

23                     6.9 

:28 

25                     6.9 

:30 

25                     6.9 

:32 

25 

7.0 

:34 

24 

:36 

25 

7.'b 

:38 

24 

7.1 

.30 


Near  the  cathode,  the  velocity  of  the  electrons  is  relatively  small,  and  the  direction 
of  their  motion  will  therefore  conform  closely  to  the  direction  of  the  strong  electric 
force.  Near  the  anode,  on  the  other  hand,  the  velocity  of  the  electrons  is  so  high  that 
the  same  force  acting  over  the  same  length  of  path  will  produce  but  little  deflection. 

G.     Fixity  of  Position  of  Focal  Spot  • 

The  focal  spot  on  the  anode  does  not  wander,  but  remains  perfectly  fixed  in  position. 
This  is  in  sharp  contrast  to  the  ordinary  Roentgen  tube  in  which  the  focal  spot  does 
move  about,  and  often  so  rapidly  as  to  be  noticeable  even  during  the  shortest  radio- 
graphic  exposures.1  The  effect  of  movement  of  the  focal  spot  is,  of  course,  to  cause  in 
the  radiograph  or  on  the  screen,  a  blurring  of  all  lines  except  those  parallel  to  the  direc- 
tion of  motion.  In  the  earlier  stages  of  exhaustion,  while  the  new  tube  is  being  operated 
with  a  relatively  poor  vacuum,  the  focal  spot  may  dance  about,  but  as  the  electrodes 
and  the  glass  become  freer  from  gas  the  motility  of  the  focal  spot  decreases  and  finally 
disappears  completely.  Its  disappearance  goes  hand  in  hand  with  the  disappearance 
of  fluorescence  of  the  glass,  discussed  in  Section  /.  Movement  of  the  focal  spot  appears 
to  be  due  to  the  action  of  positive  ions  in  disturbing  the  distribution  of  static  charge 
on  the  glass  walls  of  the  tube. 

H.     Tube  Not  Sensitive  to  Considerable  Changes  in  Gas  Pressure 
The  gas  pressure  within  the  tube  is  so  low  that  it  can  increase  several-fold,  and  ap- 
parently decrease  without  limit,  without  appreciably  affecting  the  other  characteristics. 
The  slight  effect  of  pressure  change  may  be  seen  from  the  following  experiment: 
While  one  of  the  tubes  was  being  continuously  operated  on  the  pump,  the  pressure 
as  indicated  by  a  McLeod  gauge,  decreased  from  0.113  to  0.035  micron.2     The  dis- 
charge current  passing  through  the  tube  remained  constant  at  3.1  milliampercs,  while 
the  parallel  spark  gap  backed  up  by  the  tube  changed  only  from  7.9  to  S.(>  cm.     A 
corresponding  pressure  change  in  the  case  of  an  ordinary  Roentgen  tube  would  bring 
about  an  enormous  change  in  current  and  voltage. 

/.     Capable  of  Continuous  Operation  u'ith  High  Energy  Input 

Owing  to  the  fact  that  the  tungsten  target  can  run  at  such  a  high  temperature, 
large  amounts  of  energy  can  be  continuously  radiated. 

./ .     No  Fluorescence  of  Glass 

When  operating  properly  the  tube  shows  no  fluorescence  of  the  glass  at  any  point. 
Corresponding  to  this,  there  is  an  absence  of  the  usual  strong  local  heating  of  the 
anterior  hemisphere.  The  absence  of  fluorescence  and  of  local  heating  seem  to  point 
to  the  fact  that  there  is  no  bombardment  of  the  glass  by  secondary  cathode  rays  sent 
out  from  the  target.  This  is  in  striking  contradistinction  to  what  takes  place  in  an 
ordinary  Roentgen  tube,  where,  in  the  case  of  a  platinum  target,  it  has  been  found  that 
there  are  about  three  fourths  as  many  electrons  leaving  the  target  and  going  to  the 
glass  as  secondary  cathode  rays,  as  there  are  bombarding  it  in  the  form  of  primary 
cathode  rays.  This  elimination  of  secondary  cathode  ray  bombardment  prevents  the 
production  of  a  large  part  of  the  useless  and  disturbing  Roentgen  rays  which  emanate 
from  the  glass  in  the  case  of  the  ordinary  tube. 

The  absence  of  bombardment  of  the  glass  is  of  interest  both  theoretically  and 
practically.  Other  explanations  of  the  lack  of  fluorescence  suggested  themselves  at 
first.  A  plausible  hypothesis  was  that  bombardment  took  place,  but  that  the  surface 
of  the  glass  was  much  freer  from  gas  than  it  is  in  the  ordinary  tube  and  that  this 

1  See  Dr.  Pfahler,  Fortschritte  auf  dem  Gebiete  der  Rontgenstrahlen,  18,  pp.  340-343  (1911-1912). 
*  In  the  light  of  later  experiments  it  seems  doubtful  whether  a  further  pressure  decrease,  no  matter  how  great,  wouW 
have  appreciably  affected  the  tube  characteristics. 

51 


accounted  for  the  lack  of  fluorescence.  But  the  fact  that  fluorescence  appears  so  sud- 
denly when  a  trace  of  gas  is  evolved,  coupled  with  the  fact  that  such  fluorescence  may 
appear  in  streaks  and  that  these  may  rapidly  change  their  location,  seems  to  disprove 
the  hypothesis.  It  also  seemed  possible  at  first  that  the  fluorescence  might  be  there, 
but  thatf  it  could  not  be  seen  because  of  the  strong  light  emission  from  both  filament 
and  target.  But  this  hypothesis  is  disproved  by  the  fact  that  with  filament  and  target 
at  their  highest  temperatures,  fluorescence  becomes  suddenly  strongly  visible  when- 
ever gas  is  liberated. 

The  simple  explanation  appears  to  be  based  upon  the  fact  that  the  large  number  of 
positive  ions  present  in  an  ordinary  Roentgen  tube  is  here  lacking.  The  inner  surface 
of  the  glass  becomes  strongly  negatively  charged,  when  the  tube  is  first  operated,  and 
not  being  able  to  attract  an  appreciable  number  of  positive  ions,  remains  so.  The 
presence  of  this  negative  charge  upon  the  glass  prevents  further  electrons,  either  in 
the  shape  of  primary  or  secondary  cathode  rays,  from  going  there. 

A".     Identity  of  Starting  and  Running  Voltage 

The  starting,  or  break-down  voltage  of  the  tube  is  the  same  as  the  running  voltage. 
This  is  very  different  from  the  state  of  affairs  in  the  ordinary  tube  in  which  the  break- 
down voltage  is  much  higher  than  the  running  voltage.1  The  difference  is  to  be  ex- 
plained as  follows :  In  the  ordinary  tube  the  number  of  ions  present  when  the  circuit 
is  closed  is  exceedingly  small,  being  only  that  due  to  natural  ionization  causes,  such 
as  radioactive  matter  in  the  surroundings.  After  the  discharge  circuit  is  closed,  the 
number  of  ions  increases,  by  collision,  very  rapidly,  and  the  voltage  across  the  tube 
terminals  falls  in  consequence.  In  the  case  of  the  new  tube,  on  the  other  hand,  the  full 
supply  of  electrons  is  there  the  instant  the  discharge  circuit  is  closed,  and  even  before 
this,  and  the  available  number  is  not  changed  by  the  discharge  current. 

L.  No  Heating  of  Cathode  byDischargeCurrent  and  noEvidence  of  Cathodic  Disintegration 
An  earlier  experiment  showed  that  when  a  tube  is  made  up  with  two  similar  con- 
cave tungsten  electrodes,  symmetrically  located  in  the  tube,  and  operated  on  direct 
current  at  an  ordinary  Roentgen  tube  vacuum,  the  heating  effect  at  the  cathode  is  as 
strong  as  that  at  the  anode.  Furthermore,  the  heat  evolution  at  the  cathode  is  as 
strongly  localized  as  it  is  at  the  anode.  In  fact,  it  is  impossible,  when  the  tube  is 
operating,  to  tell,  by  looking  at  it,  which  of  the  white  hot  electrodes  is  functioning  as 
anode  and  which  as  cathode.  There  is  a  very  rapid  blackening  of  the  bulb  in  such  a 
tube,  the  material  of  the  deposit  coming  evidently  from  the  cathode,  which  shows  a 
deep  and  sharply  defined  cavity  at  the  point  of  local  heating.  The  simple  explanation 
seems  to  be  that  the  cathode  is  bombarded  by  positive  ions  and  that  the  emission  of  the 
electrons  which  constitute  the  cathode  ray  stream  is  due  to  this  bombardment.  So, 
also,  the  heating  effect  and  the  cathodic  disintegration. 

At  the  higher  vacuum  and  with  the  relatively  gas-free  electrodes  of  the  new  tube 
there  is  no  evidence  of  any  bombardment  of  the  cathode.  In  the  earlier  stages  of  gas 
removal ,  when  a  discharge  can  be  made  to  pass  through  the  tube  without  the  heating 
current  in  the  filament,  the  latter  is  seen  to  be  strongly  locally  heated  by  the  discharge 
current,  as  from  bombardment  by  positive  ions.  But  when  the  exhaustion  has  been 
completed  and  the  tube  is  operated  with  the  cathode  hot,  a  voltmeter  and  ammeter 
in  the  filament  circuit  show  no  change  even  when  a  very  heavy  discharge  is  sent  through 
the  tube.  Positive  ion  bombardment,  if  it  existed  to  an  appreciable  extent,  would 
raise  the  temperature  and,  hence,  the  resistance  of  the  tungsten  filament  and  would 
therefore  be  indicated  by  the  instruments.  If  it  were  very  local  and  considerable,  it 

1  See  Dessauer,  1.  c. 

52 


would  be  further  indicated  by  a  melting  through  of  the  filament  at  the  point  in  question. 
The  resistance  change  and  local  disintegration  of  the  filament  have  been  observed  in 
only  those  cases  where  the  vacuum,  as  shown  by  other  effects,  such  as  fluorescence 
of  the  glass,  has  been  poor. 

Disintegration  of  the  cathode  would  also  manifest  itself  in  blackening  of  the  bulb. 
Even  after  running  for  several  hours,  the  deposit  on  the  bulb  is  very  slight,  and  what 
there  is  may  well  be  entirely  accounted  for  by  vaporization  of  tungsten  at  the  focal 
spot  on  the  target. 

M.     The  Target  the  Factor  Limiting  Allowable  Energy  Input 

There  is  one  limitation  with  the  new  tube.  With  a  sharp  focusing  tube  and  above 
a  certain  energy  input  the  tube  resistance  is  unstable,  dropping  suddenly  to  perhaps 
a  small  fraction  of  its  original  value,  returning  instantly  to  the  old  value  however  upon 
stopping  the  discharge  or  upon  lowering  it  to  the  limiting  value.  The  cause  of  this 
phenomenon  appears  to  be  as  follows : 

With  a  very  high  energy  input  and  sharp  focusing,  the  surface  of  the  target  melts 
at  the  focal  spot  and  volatilizes.  Owing  to  the  fact  that  this  tungsten  vapor  is  pro- 
duced at  the  focal  spot,  all  of  the  primary  cathode  rays  pass  through  it  and,  by  collision, 
ionize  it.  This  of  course  decreases  the  tube  resistance.  The  larger  the  focal  spot  the 
greater  is  the  limiting  current.  The  design  of  the  target  also  has  a  great  deal  to  do  with 
the  limiting  current  value,  as  the  face  of  a  thin  target  is  vaporized  with  a  much  lower 
energy  input  than  a  relatively  thick  one.  For  very  short  excitation  of  the  tube,  the 
limiting  energy  input  is  somewhat  larger  than  for  longer  periods ;  but  an  input  which 
can  be  carried  for  a  few  seconds  can  be  carried  indefinitely. 

The  effect  of  substituting  any  other  single  refractory  metal  for  the  tungsten  of  the 
target  will  be  to  lower  the  maximum  allowable  energy  input.  For  the  essential  prop- 
erties of  a  target  material  are:  high  density,  high  melting  point,  high  heat  conductivity, 
and  low  vapor  pressure.  Tungsten  has  a  higher  melting  point  and  lower  vapor  pres- 
sure than  any  other  metal.  Its  nearest  competitor  in  point  of  refractoriness,  tantalum, 
has  only  about  one  third  of  the  heat  conductivity.  Molybdenum  and  iridium  have 
vapor  pressures  altogether  too  high  to  entitle  them  to  consideration,  even  if  their 
melting  points  made  them  otherwise  competitors.  Osmium  has  only  about  one  half 
of  the  heat  conductivity  of  tungsten. 

The  ordinary  copper-backed  tungsten  target  would  be  very  difficult  to  exhaust 
sufficiently.  Otherwise  its  use  might  be  desirable  for  certain  classes  of  work,  as  it 
would  raise  the  maximum  allowable  instantaneous  energy  input. 

§  6.     DANGER  CONNECTED  WITH  USE  OF  TUBE 

There  has  been  in  the  old  tube  a  certain  element  of  safety  in  that  it  could  not  be 
run  continuously  with  a  very  heavy  energy  input.  The  new  tube,  even  when  focusing 
sharply,  can  be  operated,  for  example,  on  a  7-cm.  parallel  spark  gap  with  currents  as 
high  as  25  milliamperes  for  hours  at  a  time,  and  without  the  slightest  attention. 

For  most  purposes,  other  than  diagnostic  or  radiographic  work,  there  is  no  advan- 
tage in  having  the  tube  focus.  In  case  it  does  not,  the  above-mentioned  energy  input 
limitation  falls  away  and  the  tube  can  apparently  be  designed  for  any  energy  input 
whatsoever.  This  will  permit,  in  this  field,  of  the  use  of  much  greater  Roentgen  ray 
intensities  than  have  heretofore  been  realized. 

In  the  light  of  the  above,  it  will  be  seen  that  the  precautions  which  have  been 
shown  by  years  of  experience  to  be  sufficient  for  work  with  the  old  tube  are  not  neces- 
sarily sufficient  for  the  user  of  the  new  one. 

53 


§  7.     SUMMARY 

In  the  foregoing,  a  new  and  powerful  Roentgen  ray  tube  has  been  described.  It 
differs  in  principle  from  the  ordinary  type  in  that  the  discharge  current  is  purely 
thermionic  in  character.  Both  the  tube  and  the  electrodes  are  as  thoroughly  freed 
from  gas  as  possible,  and  all  of  the  characteristics  seem  to  indicate  that  positive  ions 
play  no  appreciable  role. 

The  tube  allows  current  to  pass  in  only  one  direction  and  can  therefore  be  operated 
from  either  direct  or  alternating  current. 

The  intensity  and  the  penetrating  power  of  the  Roentgen  rays  produced  are  both 
under  the  complete  control  of  the  operator,  and  each  can  be  instantly  increased  or 
decreased  independently  of  the  other. 

The  tube  can  be  operated  continuously  for  hours,  with  either  high  or  low  discharge 
currents,  without  showing  an  appreciable  change  in  either  the  intensity  or  the  pene- 
trating power  of  the  resulting  radiations. 

The  tube  in  operation  shows  no  fluorescence  of  the  glass  and  no  local  heating  of  the 
anterior  hemisphere. 

The  starting  and  running  voltage  are  the  same. 

An  article  bearing  especially  upon  the  application  of  the  new  tube  to  radiographic 
and  diagnostic  and  to  therapeutic  purposes  will  appear  shortly  in  one  of  the  Roentgen 
ray  journals. 

It  is  a  pleasure  to  me,  in  closing,  to  express  my  appreciation  of  the  services  of  Mr. 
Leonard  Dempster,  who  has  assisted  me  throughout  this  work. 


54 


PHYSICAL  INVESTIGATION  WORK  IN  PROGRESS  ON  TUBES  AND 

ACCESSORIES* 

BY  \V.  D.  COOLIDGE,   PH.D. 

The  following  is  a  continuation  of  a  paper1  given  last  year  and  is  in  the  main,  a  re- 
port of  unfinished  work. 

THE  PRODUCTION  AND  MAINTENANCE  OF  VERY  HIGH  VACUA 

The  evolution  of  the  vacuum  pump  has  been  very  rapid  in  the  last  few  years.  An 
enormous  stride  was  taken  by  Gaede  who,  in  1912,  published  the  story  of  the  so-called 
Molecular  Pump.2  Ever  since  it  became  available,  this  has  been  an  extremely  useful 
tool,  but  because  of  the  high  speed  at  which  it  runs  the  difficulty  of  keeping  it  in  good 
condition  for  continuous  operation  has  been  very  considerable.  Furthermore,  there 
was  a  limit  to  the  degree  of  exhaustion  which  could  be  attained  with  it.  For  these  rea- 
sons a  great  deal  of  interest  was  aroused  by  Gaede's  announcement,  in  1915,  of  his  so- 
called  Diffusion  Pump.3  With  this  device  it  was  possible,  theoretically  at  least,  to  re- 
move the  very  last  molecule  from  the  space  to  be  exhausted.  There  were,  furthermore, 
no  moving  parts  in  the  device.  By  the  application  of  heat,  mercury  was  caused  to  boil 
and  the  column  of  mercury  vapor,  upon  rising,  passed  by  a  narrow  slit  in  the  wall  of  the 
container.  This  slit  was  in  communication  with  the  space  to  be  exhausted,  and  any  gas 
molecules  coming  from  this  space,  and  diffusing  through  the  slit,  were  carried  along  by 
the  ascending  column  of  mercury  vapor.  The  mercury  vapor  being  condensed  in  the 
upper  part  of  the  device,  ran  back  as  liquid  mercury.  The  gas  carried  up  by  the  mer- 
cury vapor  was  removed  by  an  ordinary  air  pump.  The  one  serious  limitation  in  the 
device  was  the  speed  of  operation,  which  was  considerably  less  than  that  of  the  "molec- 
ular" pump. 

Dr.  Langmuir,  of  our  laboratory,  has  developed  a  new  form  of  mercury-vapor 
pump — the  so-called  Condensation  Pump.4  It  produces  the  same  high  vacuum  as  the 
"  Diffusion  Pump,"  operates  more  rapidly  and  is  easier  to  construct.  Its  mode  of  oper- 
ation may  be  seen  by  referring  to  Fig.  1  in  which  A  represents  a  glass  bulb  containing 
mercury;  B  a  glass  tube  leading  into  a  larger  glass  tube,  C;  D  a  tube  leading  to  the  vessel 
to  be  evacuated,  and  E  a  water  jacket. 

The  mercury  in  A  is  heated  and  kept  boiling  by  a  small  electric  stove  underneath. 
Waste  of  heat  is  reduced  by  lagging  A  and  B  with  asbestos.  Between  D  and  the  vessel 
to  be  exhausted  is  a  liquid-air  trap  which  serves  to  condense  any  mercury  vapor  com- 
ing from  the  pump. 

In  operation,  a  vigorous  blast  of  mercury  vapor  is  maintained  down  through  the 
open  end  of  B  into  C.  Gas  molecules,  coming  through  D  from  the  vessel  to  be  ex- 
hausted, diffusing  into  the  mercury  blast  are  carried  downward  by  it  to  the  lower  end  of 
C,  from  which  point  they  are  removed  by  an  ordinary  air  pump.  The  tube  C  is  cooled 
by  the  water  jacket,  the  mercury  vapor  condenses  on  its  sides  and  flows  back  to  A . 

With  the  help  of  the  condensation  pump,  the  evacuation  of  the  space  in  an  X-ray 
tube  becomes  a  simple  matter. 

1  Am.  Journ.  of  Rocntgenology,  December,  1915. 

=  W.  Gaede,  Ber.  d.  d.  phys.  Ges.,  15,  775-787  (1912). 

*  Ann.  d.  Physik,  46,  357-392  (1915). 

4  Phys.  Rev.,  8,  p.  48.  July  (1916). 

*Read  at  the  Seventeenth  Annual  Meetirg  of  the  Arr.erican  Rr.entgen  Ray  Society,  Chicago,  Fept.  27-30,  1916. 

Copyright  by  the  American  Journal  of  Roentgenology,  1917. 


Oil 


The  manner  in  which  gas  is  removed  from  the  metal  parts  has  already  been  de- 
scribed.5 

The  glass  is,  under  certain  conditions,  a  much  more  troublesome  source  of  gas. 
Whenever  a  tube  is  heated  to  a  higher  temperature  than  it  had  whi  e  connected  to  the 
pump,  water  vapor  and  carbon  dioxide  are  given  up  to  the  space  from  the  inner  surface 
of  the  glass.  This  situation  is  helped  by  heating  the  tube  almost  to  the  collapsing 
point  while  it  is  connected  to  the  pump.  That  this  does  not  completely  take  care  of  the 
problem,  however,  is  shown  by  the  experience  we  have  had  in  attempting  to  greatly  re- 
duce the  size  of  tubes. 

With  a  9.5-cm.  (3^-inch)  bulb  we  have  found  that  it  takes  an  energy  input  of  1200 
watts,  continuously  applied,  to  cause  the  glass  to  collapse.  (The  tube  in  this  experi- 


Di  awing  of  Langmuir's  New  Form  of  Mercury-vapor  Pump — 
the  So-called  Condensation  Pump 

ment  had  a  regular  solid  tungsten  target.  The  tube  was  freely  exposed  to  the  air,  and 
was  in  a  metal  box  1 12  cm.  high,  56  cm.  wide  and  1 14  cm.  long.  There  was  no  forced  cir- 
culation of  air.)  When  this  tube  was  exhausted  in  the  same  manner  as  the  17.8-cm. 
(7-inch)  tube  now  on  the  market,  we  found  that  an  energy  input  of  360  watts,  when  con- 
tinuously applied  for  1 Y^  to  2  minutes,  would  cause  the  evolution  of  sufficient  gas  from 
the  glass  to  render  the  tube  useless  until  re-exhausted.  After  re-exhaustion,  upon  again 
testing  the  tube,  it  was  found  that  the  360  watts,  continuously  applied  for  a  couple  of 
minutes,  would  once  more  render  re-exhaustion  necessary.  (For  ordinary  roentgeno- 
graphic  work,  the  tube  was  all  right,  as  this  did  not  heat  the  glass  to  too  high  a  tem- 
perature.) 

Greatly  extending  the  exhaust  time  on  the  pump  did  not  help  materially,  and  the 
long  continued  operation  of  the  tube,  with  continuous  evolution  of  gas,  resulted  in  a 
very  considerable  metallic  deposit  on  the  bulb. 


5  Ibid.  6,  416-417  (1913). 


56 


In  the  water-cooled  tube,  we  had  found  that  the  use  in  the  electrode  construction 
of  some  metal  which  undergoes  strong  cathodic  disintegration,  such  as  copper  for  ex- 
ample, was  very  helpful.  When  the  vacuum  became  sufficiently  impaired,  the  copper 
was  subjected  to  positive  ion  bombardment  and  was  thereby  spattered  on  to  the  glass 
walls  of  the  tube.  By  this  process,  gas  molecules  were  removed  chemically  and 
mechanically  from  the  space  and  the  vacuum  was  in  this  manner  automatically  im- 
proved by  the  electrical  operation  of  the  tube.  The  experiment  was,  therefore,  tried 
with  the  9.5-cm.  tube,  of  introducing  copper  into  the  cathode  structure  (as  a  major  por- 
tion of  the  focusing  device) .  This  did  not  appear  to  help  the  situation  at  all.  The  rea- 
son is  probably  to  be  found  in  the  fact  that  in  the  air-cooled  tube  the  bulb  gets  very  hot, 
while  it  does  not  in  the  water-cooled  tube.  Much  of  the  gas  which  is  trapped  by  the 
copper  deposited  on  the  bulb  is  doubtless  released  again,  if  the  bulb  is  allowed  to  get  hot. 

Thinking  that  we  might  possibly  get  help  from  it,  we  introduced  some  metallic  tho- 
rium powder  into  the  bulb  of  one  of  the  small  tubes.  The  experiment  was  successful 
and  we  find  that  the  bulb  of  such  a  tube  can  be  heated  to  the  collapsing  point  without 
troublesome  impairment  of  the  vacuum.  Metallic  zirconium  powder  can  also  be  used. 
Several  other  metals  were  tried,  such  as  calcium,  titanium  and  tantalum,  but  none  of 
these  proved  helpful.  (Calcium  apparently  has  too  high  a  vapor  pressure.)  It  seems 
most  probable  that  the  rare  earth  metals,  thorium  and  zirconium,  help  in  maintaining  a 
vacuum  by  reacting  with  the  gases  given  off  from  the  glass,  and  that  the  solid  com- 
pounds so  formed  have  an  exceedingly  low  decomposition  pressure,  even  at  the  tem- 
perature at  which  lime  glass  softens. 

We  see,  then,  in  the  use  of  these  metals  one  way  of  increasing  the  allowable  energy 
input  of  a  tube  of  a  given  size. 

Another  method  which  suggested  itself  was  to  make  the  tube  entirely  of  fused 
quartz.  This  has  been  tried  and  found  successful.  Such  a  tube,  only  6  centimeters 
in  diameter,  has  been  successfully  operated  continuously  with  2000  watts  and  this  with- 
out the  help  of  forced  air  circulation. 

At  the  moment,  another  method  which  does  not  involve  the  use  of  the  rare  earth 
metals  or  of -fused  quartz  is  being  tried  out.  It  consists  in  first  heating  the  tube  almost 
to  the  collapsing  point,  while  connected  to  the  pump,  and  then  letting  in  specially  puri- 
fied nitrogen  gas  so  as  to  have  the  same  pressure  within  and  without.  The  temperature 
is  then  raised  to  just  short  of  the  point  at  which  the  glass  will  sag  under  its  own  weight. 
This  is  nearly  200  deg.  C.  higher  than  the  bulb  will  stand  when  evacuated.  By  the  al- 
ternate application  of  the  high  temperature  treatment,  with  and  without  vacuum, 
several  times,  we  find  that  the  inner  surface  of  the  glass  is  greatly  improved.  By  this 
method  we  have  already  succeeded  in  producing  a  glass  tube  with  a  9.5-cm.  bulb  which 
can  be  continuously  operated  with  as  much  as  600  watts,  and  this  without  forced  air 
circulation.  More  experimental  work  is  to  be  done  on  this  method  to  see  if  it  cannot  be 
so  modified  as  to  enable  us  to  safely  let  the  tube  get  still  hotter.  (This  tube  in  which 
we  are  now  able  to  use  600  watts  continuously  without  forced  air  circulation  will  not 
collapse  with  less  than  1200  watts.) 

CATHODE  DESIGN 

To  get  the  greatest  possible  X-ray  intensity  from  a  focal  spot  of  given  size  (and  this 
must  always  be  the  desideratum  in  diagnostic  work) ,  the  energy  delivered  to  the  focal 
spot  must  be  as  uniformly  distributed  over  it  as  possible.  The  size  of  the  focal  spot 
and  the  energy  distribution  are  determined  by  the  cathode  design,  the  size  and  shape  of 
anode,  the  distance  between  anode  and  cathode,  and  the  size  of  bulb. 


Widely  different  focal  spots  can  be  produced  with  the  same  design  of  filament 
spiral,  the  same  design  of  anode,  the  same  size  of  bulb  and  the  same  distance  between 
filament  spiral  and  anode,  by  merely  changing  the  design  of  the  focusing  device.  This 
may  be  seen  by  referring  to  Fig.  2,  which  shows  three  different  cathode  assemblies. 
The  filament  spirals  are  exactly  alike  in  all  three,  but  the  focusing  devices  are  radically 
different.  To  the  right  of  each  cathode  diagram  is  a  Roentgen  ray  pinhole  camera 
picture  of  the  focal  spot,  made  with  that  cathode.  The  anode  used  was  in  all  cases 
the  standard  45  deg.  angle  type  and  the  distance  from  anode  to  filament  spiral  was  in 
all  cases  the  same. 

Fig.  3  shows  how  the  focal  spot  changes  when,  all  other  factors  remaining  the  same, 
the  location  of  the  focusing  device  with  respect  to  the  filament  spiral  is  changed.  The 


diagram  shows  three  different  settings  of  an  adjustable  cathode  (these  settings  were 
made  by  merely  tapping  the  tube  while  it  was  held  in  an  inclined  position)  and,  to  the 
right  of  each,  a  pinhole  camera  picture  of  the  focal  spot,  made  with  that  setting. 

Finally,  Fig.  4  shows  how  the  focal  spot  changes  with  the  distance  between  filament 
spiral  and  anode.  In  this  case,  the  setting  of  the  adjustable  cathode  was  held  constant 
and  the  position  of  the  movable  anode  was  changed. 

The  problem  of  getting  the  best  focal  spot  of  a  given  size  for  a  given  size  of  bulb  has 
finally  resolved  itself  into  making  up  an  experimental  tube  in  which  the  distance  be- 
tween anode  and  filament  spiral  can  be  varied  at  will  and  in  which  the  location  of  the 


filament  with  respect  to  the  focusing  device  can  also  be  varied  at  will.  The  con- 
struction of  such  a  tube  is  shown  in  Fig.  5. 

The  focusing  device,  d,  which  is  in  this  case  a  hemispherical  bowl,  is  attached  to  a 
sleeve  which,  by  tapping,  can  be  made  to  slide  on  /",  the  molybdenum  tube  in  which  the 
filament  spiral  is  fastened. 

The  anode  also  can  be  moved  by  holding  the  tube  in  a  vertical  position  and  rapping 
the  lower  end  smartly  against  a  firmly  supported  piece  of  soft  wood.  This  causes  the 
iron  support-tube  to  slide  in  the  glass  tube,  b. 

Focal  spot  pictures  are  made  as  each  variable  is  changed  separately,  and  an  in- 
spection of  these  shows  which  setting  of  focusing  device  and  which  distance  from  fila- 
ment spiral  to  anode  gives  the  best  focal  spot. 


METAL  TUBE 

In  February  of  last  year,  Prof.  Zehnder6  published  an  article  entitled  "Eine  gefahr- 
lose  metallische  Rontgenrohre "  (A  Safe  Metallic  Roentgen  Ray  Tube).  A  somewhat 
similar  metal  tube  has  also  been  used,  while  connected  to  the  pump,  by  Siegbahn.7 

•  L.  Zehnder,  Electrotech,  Zttchr.,  Heft  5.  pp.  49-50,  Feb.  4  (1915). 

7  For  a  description  of  this  tube,  see  the  Inaugural  Dissertation  of  Ivar  Malmer  entitled  "  Untersuchungen  viber  die 
Hoc-hfrequenzspectra  der  Elemente."  Lund  (1915). 

59 


There  seem  to  be  many  reasons  why  such  a  tube  as  that  described  by  Zehnder  may 
be  impracticable,  but  he  has,  at  least,  stated  a  good  problem.  There  are  some  very 
great  advantages  which  might  be  derived  from  a  metal  tube,  if  it  can  be  so  constructed 
that  it  can  be  successfully  operated. 

Until  we  had  developed  a  tube  with  a  water-cooled  anode,8  we  had  doubts  as  to 
whether  it  would  ever  be  possible  to  secure  and  maintain  in  a  metal  tube  a  vacuum 
sufficiently  high  for  a  tube  with  a  pure  electron  discharge.  Our  experience  with  the 
water-cooled  tube,  however,  encouraged  us  to  try  the  metal  tube.  The  first  attempts 
were  not  at  all  promising,  as  we  seemed  to  have,  in  the  large  amount  of  metal,  a  never- 
failing  source  of  gas,  too  much  for  satisfactory  operation  even  while  the  tube  was  con- 
nected to  the  pump.  Thinking  that  the  difficulty  might  possibly  be  due  to  oxidation 
of  the  metal,  we  filled  the  tube  with  purified  hydrogen  and  heated  it.  The  hydrogen, 
containing  water-vapor  from  the  reaction  which  had  taken  place,  was  then  pumped  out 
and  fresh  hydrogen  admitted.  This  cycle  of  operations  was  repeated  several  times. 
After  such  treatment,  it  was  found  that  the  tube  could  be  satisfactorily  exhaused  and 
that  it  would  maintain  its  vacuum  upon  being  sealed  off  from  the  pump. 

Another  very  serious  difficulty  which  we  experienced  was  the  puncturing  and  tear- 
ing of  the  glass,  even  at  low  operating  voltages,  in  the  neighborhood  of  where  the  glass, 


F&5. 

used  to  insulate  the  cathode,  is  sealed  on  to  the  metal  portion  of  the  tube.  This  diffi- 
culty was  finally  obviated  by  the  use  of  an  inner  metal  sleeve  which  is  attached  to  the 
metal  body  of  the  tube  and  extends  for  some  distance  beyond  the  glass  seal.  The  help- 
ful action  of  this  sleeve  is  probably  due  to  the  fact  that  it  electrostatically  prevents 
electrons  from  getting  to  the  inner  surface  of  the  glass  near  the  seal. 

Fig.  6  shows  a  cut  of  such  a  tube.  In  the  figure,  a,  the  body  of  the  tube,  is  of  copper. 
The  cathode,  /,  projects  into  a  and  is  supported  and  insulated  from  it  by  the  glass  tube 
c.  b  is  a  flaring  piece  of  thin-walled  platinum  tubing  which  is  silver-soldered  at  the 
small  end  to  the  copper  tube  and  sealed  at  the  large  end  to  c.  The  hot  cathode,  /,  is 
supported  by  means  of  the  copper  tube  e,  which  at  the  outer  end  is  enlarged  and  fitted 
tightly  on  to  the  glass  tube  d.  Of  the  two  copper  wires  carrying  current  in  to  the  fila- 
ment, the  one  marked  h  is  metallically  connected  to  e,  while  the  other  is  insulated,  g  is  a 
window  of  thin  platinum  which  also  serves  as  target.  It  is  so  thin  that  a  considerable 
fraction  of  the  Roentgen  rays  produced  on  the  side  next  to  the  cathode  pass  through 
it  and  are  available  for  use  on  the  other  side. 

In  operation,  the  metal  body  of  the  tube  is  connected  to  the  positive  terminal  of  the 
generator. 


8  This  Journal  for  December,  1915,  pp.  7-10. 

60 


In  most  of  the  experimental  work  which  has  been  done  on  the  tube  up  to  this  time, 
there  has  been  no  window  put  in  for  the  exit  of  the  X-rays.  In  the  absence  of  a  window, 
and  with  a  current  of  200  milli-amperes  and  a  4-inch  spark  gap,  there  was  no  visible 
illumination  of  an  "Astral"  screen  held  close  to  the  tube. 

Rays  may  be  taken  out  of  such  a  tube  in  two  ways,  either  through  a  target  of  thin 
foil  in  the  end  of  the  tube  or  from  a  massive  45  deg.  angle  target  and  out  through  a  win- 
dow in  the  side.  The  tube  has  been  cooled  by  means  of  a  metal  jacket,  slipped  over  a, 
through  which  water  was  kept  flowing. 

If  it  can  be  brought  to  a  satisfactory  state  of  development,  such  a  tube  would  have, 
among  others,  the  following  advantages : 

(1)  It  can  be  so  designed  that  no  direct  rays,  other  than  the  narrow  pencil  desired, 
can  leave  it.    The  walls  would  either  be  made  so  thick  that  no  appreciable  amount  of 
radiation  could  get  through  them,  or  a  protecting  jacket  of  lead  or  some  other  metal  of 
high  atomic  weight  would  be  slipped  on  the  outside.    This  means  that  the  present  com- 
bination of  a  seven-inch  tube  and  protective  holder,  weighing  perhaps  25  or  30  Ibs., 
could  be  replaced  by  a  metal  tube  of  higher  power  and  more  complete  protection, 
weighing  perhaps  not  more  than  half  a  pound.    This  would  make  it  possible  to  simplify 
fluoroscopic  apparatus.    The  small  size  of  the  metal  tube  would  also  be  a  convenience 
in  therapeutic  work. 

(2)  The  tube  can  be  built  with  a  diaphragm,  in  the  form  of  a  metal  cone,  projecting 
in  almost  to  the-  focal  spot,  so  as  to  effectively  keep  within  the  tube  all  rays  except  those 
emanating  from  the  focal  spot. 


(  \               —  •         ^^1 

—     ^                                  'h 

v       ^e     i                       

1 
a  —  ~^f 

E 

/  -H  »  

-^-L^l    \ 

Fig. 6. 
Diagram  One  Third  Natural  Size  of  New  Tube  with  Inner  Metal  Sleeve.      (See  text) 

(3)  It  lends  itself  to  very  effective  cooling  of  the  anode  and,  hence,  to  high  power 
work. 

(4)  The  type  in  which  the  rays  are  taken  through  a  thin  metal  foil  acting  as  a  target 
in  the  end  of  the  tube  should  be  capable  of  development  for  insertion  into  body  cavities 
for  internal  therapy.     The  source  of  X-rays  would  be  in  the  extreme  end  of  the  tube 
and  the  portion  inserted  would  be  all  metal  and  connected  to  earth.     There  would 
therefore  be  no  danger  to  the  patient  from  either  breaking  glass  or  electric  shock. 

HIGH-POWER,  HIGH-VOLTAGE  OUTFIT  FOR  EXPERIMENTAL  THERAPY 
A  special  transformer  has  been  developed  for  use  with  a  water-cooled  tube.    In  this 
outfit  the  tube  rectifies  its  own  current  and  there  is  no  mechanical  rectifying  switch. 
The  transformer  is  designed  to  supply  continuously  (by  the  hour  if  necessary)  a  max- 
imum current  of  60  milliamperes  at  a  maximum  voltage  of  150,000  (effective). 

The  installation  of  such  an  outfit  brings  new  problems  with  it.  The  most  vital  of 
these  is  the  protection  of  both  operator  and  patient  from  both  X-rays  and  electric 
shock.  Mr.  Perkins  has  worked  out  all  of  the  details  of  the  installation  and  will  give 
a  full  description  of  the  outfit  in  the  near  future  in  the  General  Electric  Review. 

With  this  outfit  it  should  be  possible  to  test  out  the  effect  of  higher  voltage  than  is 
now  being  used,  and,  at  the  same  time,  to  resort  to  much  heavier  filtering  than  is  now 
practicable. 

61 


A    NEW    DEVICE    FOR    RECTIFYING    HIGH    TENSION    ALTERNATING 

CURRENTS* 

THE  KENOTRON 

BY  DR.  SAUL  DUSHMAN 

INTRODUCTION 

The  emission  of  negatively  charged  corpuscles  or  electrons  from  heated  metals  may 
be  illustrated  by  the  following  arrangement:  In  an  ordinary  lamp  bulb  containing  a 
tungsten  or  carbon  filament  there  is  also  sealed-in  a  metal  plate.  After  the  lamp  is 
well  exhausted  it  is  observed,  on  charging  the  filament  negatively  (making  it  cathode) 
with  respect  to  the  plate,  that  a  current  passes  across  the  vacuous  space.  If  the  fila- 
ment is  charged  positively  this  current  disappears.  Furthermore,  the  magnitude  of 
this  electron  emission  (thermionic  current)  from  the  heated  cathode  increases  with 
increase  in  the  temperature  of  the  filament. 

This  effect  had  been  observed  by  Edison  and  was  more  fully  investigated  in  the  case 
of  carbon  lamps  by  Fleming.1  In  view  of  the  unilateral  conductivity  possessed  by  such 
an  arrangement  as  that  described  above,  Fleming  applied  it  as  an  "electric  valve"  to 
rectify  electric  oscillations  such  as  are  obtained  from  a  "wireless"  antenna,  and,  there- 
fore, render  it  possible  for  these  oscillations  to  affect  a  galvanometer  or  telephone.2 

That  the  current  from  a  hot  cathode  in  an  exhausted  bulb  is  due  to  a  convection  of 
electrons,  that  is  of  negatively  charged  corpuscles  having  a  mass  which  is  about 
I/  1800th  of  that  of  a  hydrogen  atom,  may  be  shown  by  deflecting  the  current  in 
magnetic  and  electrostatic  fields  and  determining  the  ratio  e/m.  Another  method  is 
that  described  below  and  which  depends  upon  the  space  charged  produced  by  the 
electrons  under  certain  conditions. 

The  relation  between  thermionic  current  and  temperature  of  cathode  was  further 

investigated  by  Richardson  and  he  found  that  in  all  cases  the  relation  could  be  accu- 

_& 

ratelv  represented  bv  an  equation  of  the  form, 

(1) 


where  a  and  b  are  constants  for  the  particular  metal  and  i  is  the  saturation  thermionic 
current  per  unit  area  at  the  absolute  temperature  T. 

Subsequent  experiments,  however,  by  other  investigators  tended  to  throw  much 
doubt  upon  the  actual  existence  of  a  pure  electron  emission  from  a  heated  metal  in  a 
good  vacuum.  It  was  found  that  different  gases  affected  the  values  of  the  constants  o 
and  b  to  an  immense  extent,  so  that  at  the  same  temperature  the  thermionic  current 
obtained  varied  over  a  very  wide  range.  Furthermore,  it  seemed  that  the  greater  the 
precaution  taken  to  attain  a  high  vacuum,  the  smaller  the  thermionic  currents  obtained, 
and  the  conclusion  was  drawn  that  in  a  "perfect"  vacuum  the  thermionic  currents 
would  disappear  altogether.  In  fact,  the  view  generally  held  until  the  past  year  by  the 
German  physicists  and  by  quite  a  few  English  physicists  was  that  the  thermionic  cur- 
rents were  due  to  chemical  reactions  in  a  gas  layer  at  the  surface  of  the  heated  metal. 
and  that,  therefore,  there  was  no  justification  for  believing  in  the  existence  of  a  pure 
electron  emission  per  ipse  from  a  heated  metal. 

»  Proc.  Roy.  Soc.,  London..  47,  122  (1890). 

1  Proc.  Roy.  Soc..  Lond..  7.J,  476  (190.3).  See  also  ].  A.  Fleming.  "Principles  of  Electric  Wave  Telegraphy  an.l 
Telephony,"  pp.  477-482  (second  edition). 

•Copyright.  1915,  by  General  Elfftric  Ret  lev. 

63 


This  subject  was  taken  up  in  the  Research  Laboratory  of  the  General  Electric  Com- 
pany, by  Dr.  Irving  Langmuir,  and  he  found  that  in  the  case  of  heated  tungsten  fila- 
ments the  electron  emission  at  constant  temperature  increased  as  the  vacuum  im- 
proved until  a  constant  value  was  attained  which  varied  with  the  temperature  in  ac- 
cordance with  Richardson's  equation.  Dr.  W.  D.  Coolidge  applied  this  fact  to  the  con- 
struction of  a  hot  cathode  Roentgen  ray  tube1  in  which  electrons  are  produced  from  a 
heated  filament  in  a  highly  exhausted  bulb.  A  tungsten  target  is  used  as  anode  and  by 
applying  very  high  voltages  (50,000  to  100,000)  the  electrons  are  given  velocities  great 
enough  to  produce  very  penetrating  X-rays  when  they  strike  the  target. 

During  the  last  few  years  Dr.  Langmuir  has  carried  out  a  detailed  investigation  of 
the  whole  subject  of  electron  emission  from  heated  metals  and  the  results  obtained 
have  led  to  a  large  number  of  interesting  and  highly  important  applications. 

While  a  complete  summary  of  these  applications  will  be  presented  by  Dr.  Langmuir 
at  a  future  meeting  of  the  Institute  of  Radio  Engineers,  it  has  been  considered  advisable 
to  publish  a  preliminary  account  of  one  important  application  of  hot  cathode  tubes 
in  the  development  of  which  the  writer  has  been  interested.  This  concerns  the  appli- 
cation to  the  rectification  of  high  tension  alternating  currents. 

THE  HOT  CATHODE  RECTIFIER 

As  mentioned  above,  the  fact  that  an  exhausted  tube,  containing  two  electrodes, 
one  of  which  is  heated  by  some  external  source,  acts  as  a  rectifier,  has  been  known  for 
a  number  of  years.  But  difficulties  were  met  with  in  the  way  of  applying  this  practi- 
cally. The  magnitude  of  the  current  obtained  was  apt  to  vary  quite  erratically, 
especially  with  slight  variations  in  degree  of  vacuum.  Furthermore,  in  the  types  of  hot 
cathode  rectifiers  exhausted  by  ordinary  methods,  the  electron  emission  is  accompanied 
by  a  blue  glow.  This  glow  becomes  more  and  more  pronounced  the  higher  the  voltage 
at  which  the  rectifier  is  operated,  and  it  is  found  that  under  these  conditions  the  cathode 
gradually  disintegrates  so  that  the  rectifier  becomes  inoperative. 

An  explanation  of  these  phenomena  gradually  developed  as  a  result  of  the  above 
mentioned  investigations  on  thermionic  currents  in  high  vacua.  It  was  perceived  that 
the  blue  glow  is  due  to  the  presence  of  positively  charged  gas  molecules  (ions) ,  and  that 
the  disintegration  of  the  cathode  is  due  to  bombardment  by  these  positive  ions  moving 
with  high  velocity.  But  when  the  vacuum  is  made  as  perfect  as  possible,  the  conduc- 
tion occurs  only  by  means  of  electrons  emitted  from  the. hot  cathode,  and  there  is  no 
evidence  whatever  of  any  blue  glow  or  other  forms  of  gaseous  discharge.  Thus,  while 
it  had  previously  been  considered  that  a  certain  amount  of  gas  is  absolutely  essential  to 
obtain  conduction  from  a  hot  cathode,  and  the  presence  of  blue  glow  was  taken  to  be  a 
necessary  accompaniment  of  conduction  in  such  cases,  it  was  found  that  by  adopting 
certain  methods  of  treatment  and  the  use  of  high  vacua,  a  hot  cathode  rectifier  could  be 
constructed  in  which  all  of  the  difficulties  discussed  above  are  avoided. 

Special  methods  have  been  developed  for  treating  all  metal  parts  and  glass  walls  so 
that  they  are  made  as  free  of  gas  as  possible.  A  Gaede  molecular  pump  in  series  with 
two  other  pumps  is  used  to  evacuate  the  tubes.  It  has  been  shown  by  the  writer2  that 
by  using  this  arrangement  together  with  a  liquid  air  trap  inserted  between  rectifier  and 
molecular  pump,  it  is  possible  to  attain  a  vacuum  as  high  as  5X10-7  mm.  of  mercury. 
At  this  pressure  the  mean  free  path  of  an  electron  is  so  great  that  the  chance  of  its  collid- 
ing with  any  gas  molecules  and  thus  forming  ions  by  collision  is  reduced  to  a  minimum. 

In  the  Coolidge  X-ray  tube  there  is  no  difficulty  in  obtaining  such  a  good  vacuum 
that  no  gaseous  discharge  occurs  even  when  150,000  volts  is  applied  across  the  elec- 

1  W.  D.  Coolidge,  Phys.  Rev.,  Dec.,  1913. 

2  S.  Dushman,  Phys.  Rev.,  April,  1914.     The  complete  paper  will  appear  very  shortly  in  the  same  journal. 

64 


trodes.  There  appears  to  be  no  limit  to  the  voltage  for  which  the  tube  may  be  con- 
structed except  that  due  to  electrostatic  strains.  A  further  discussion  of  this  point  is, 
however,  reserved  for  a  subsequent  section. 

ELECTRON  EMISSION  IN  HIGH  VACUUM 

Regarding  the  difficulty  of  obtaining  constant  values  for  the  thermionic  currents  at 
given  temperatures,  it  has  already  been  mentioned  that  in  a  sufficiently  good  vacuum 
the  results  obtained  are  perfectly  definite  and  reproducible.  In  the  case  of  tungsten  in 
a  "perfect"  vacuum  the  value  of  the  constants  a  and  b  in  the  Richardson  equation  are 
23.6X  10~9  and  52500  respectively,  where  i  is  measured  in  milliamperes  per  square  cen- 
timeter.' 

Using  these  constants,  the  values  of  i  calculated  for  different  values  of  T  are  as 
given  in  the  following  table : 

TABLE  I 


r 

i/cnr* 

2000 

4.2  milliamps. 

2100 

15.1 

2200 

48.3 

2300 

137.7 

2400 

364.8 

2500 

891.0 

2600 

2044.0 

In  Fig.  1  these  results  have  been  plotted  on  semi-logarithmic  paper.  Plotting  di- 
rectly values  of  i  against  those  of  T  one  obtains  a  curve  of  the  form  shown  in  Fig.  2.2 

SPACE  CHARGE  EFFECT 

It  was  observed  by  Langmuir  that  in  addition  to  this  temperature  limitation  the 
electron  current  may  be  also  limited  by  space  charge.  With  a  low  potential  difference 
between  the  electrodes  the  phenomena  observed  are  as  follows : 

As  the  temperature  of  the  cathode  increases,  the  electron  emission  increases  at  first 
in  accordance  with  the  equation  of  Richardson.  However,  above  a  certain  tempera- 
ture this  current  becomes  constant ;  further  increase  in  temperature  does  not  cause  any 
corresponding  increase  in  thermionic  current.  The  temperature  at  which  this  limita- 
tion occurs  increases  with  increase  in  anode  potential.  The  curves  shown  in  Fig.  2 
illustrate  this  very  well.  They  represent  the  results  observed  when  the  thermionic  cur- 
rent was  measured  from  a  10-mil  tungsten  filament  situated  along  the  axis  of  a  cylin- 
drical anode  7.62  cm.  long  and  1 .27  cm.  in  radius.  Thus,  with  a  potential  difference  of 
55.5  volts,  the  electron  emission  increased  according  to  the  equation  of  Richardson  un- 
til a  temperature  of  about  2300  deg.  K  was  attained.  With  further  increase  in  tempera- 
ture, the  thermionic  current  remained  absolutely  constant.  But  when  the  voltage  was 
increased  to  S7.5,  the  thermionic  current  continued  to  increase  up  to  2350  deg.  K.  With 
a  voltage  of  129,  the  increase  in  thermionic  current  was  observed  up  to  2400  deg.  K. 

This  effect  (which  is  observed  only  in  extremely  .good  vacua)  is  due  to  the  existence 
of  a  space  charge  produced  by  the  emitted  electrons.  In  other  words,  the  electrons 
emitted  from  the  hot  cathode  produce  an  electrostatic  field  which  tends  to  prevent  .the 

1  I.  Langmuir,  Physikal.  Zeit.,  15,  516  (1914). 

'-  S.  Dushman.  Phys.  Rev. ,4,  121  (1914).  The  area  of  the  hot  filament  used  was  0.61  cm"2.  The  experiments  from 
which  this  curve  was  plotted  were  performed  some  time  ago.  Both  the  degree  of  vacuum  attained  and  the  accuracy  of 
temperature  determination  were  not  as  good  as  that  obtained  in  measuring  the  values  of  a  and  b  given  above.  When  it  is 
considered  that  an  error  of  25  degrees  in  the  determination  of  the  temperature  at  2400  deg.  K.  is  sufficient  to  account  for 
the  difference  between  these  values  of  the  constants  and  those  given  in  the  curve,  the  discrepancy  does  not  appear  so  great . 

65 


motion  of  any  more  electrons  toward  the  anode.    As  the  positive  potential  on  the  latter 
increases,  more  and  more  electrons  are  permitted  to  reach  the  anode. 

From  theoretical  considerations  it  was  deduced  by  Langmuir  that  the  thermionic 
current  ought  to  increase  with  the  three-halves  power  of  the  voltage  (until  the  satura- 
tion current  as  defined  by  the  Richardson  equation  is  attained),  that  is,  for  electrodes 
of  any  shape,  the  space  charge  current 

;  —k    y$  /o\ 

^s     K.   v  ^z; 

Where  V  denotes  the  potential  difference  and  k  is  a  constant  depending  on  the  shape  of 
the  electrodes,  their  area  and  the  distance  apart. 

For  the  case  of  a  heated  filament  in  a  concentric  cylindrical  anode  (infinite  length) 

. —        i —    ^Ts  «b 

.  _2V2     e  V1 

9     \m7  (3) 

Where  is  is  the  thermionic  current  per  unit  length  and  r  is  the  radius  of  the  anode. 
Converting  into  ordinary  units  (milliamperes  and  volts)  this  equation 

14.6.  . 


becomes 


(4) 


60C 

7~ 

*xc 
^ooo 

toe 

IOC 

eoc 

100 
60 

/ 

/ 

E 

1 

/ 

ir 
k 

/ 

/ 

f? 

/ 

60 
*0 

K 

10 
B 
6 

•» 

e 

/ 

i 

*~y 

/Electron,  Emission  from  "Tungsten 
Calculated  from  Equation 

i--mjg?es=Z3.6»iofre-  J*T92- 
/ 

/ 

7 

/ 

/ 

I 

o 

/ 

j 

/ 

/ 

i, 

600 

/ 

/- 

» 

5 

WQ 

/ 

/ 

i 

/ 

/ 

Deat 

telvin. 

The  data  shown  in  Fig.  2,  which 
were  obtained  in  the  course  of  an  in- 
vestigation carried  out  by  the  writer, 
are  in  full  accord  with  the  results  calcu- 
lated from  this  equation.  Substituting 
for  r  the  value  1.27  cm.,  and  noting 
that  the  actual  length  of  cylinder  used 
was  7.62  cm.,  the  values  of  the  constant 
factor  as  obtained  from  the  observed 
space  charge  currents  for  different 
voltages  do  not  differ  by  more  than  2 
per  cent  from  14.6.1 

In  the  case  of  a  heated  tungsten 
plate  parallel  to  another  plate,  the 
space  charge  current  per  sq.  cm.,  in 
mi'liamperes,  is 

V* 


«*  =2.32X1(T>X- 


(5) 


Fig.  1.     Electron  Emission  from  Tungsten  in  a 
"Perfect"  Vacuum 


where  %   is  the  distance  between  the 
plates  in  centimeters. 

It  ought  to  be  observed  that  up  to 
a  point  at  which  the  diameter  of  the 
filament  amounts  to  about  five  per  cent 
of  the  diameter  of  the  anode  cylinder, 
or  of  the  distance  between  the  plates, 
the  space  charge  voltage  is  independent  of  the  actual  diameter  of  the  filament. 

The  thermionic  current  from  a  hot  cathode  may,  therefore,  be  limited  either  by  tempera- 
ture or  by  space  charge.  With  a  given  temperature  of  the  cathode,  the  thermionic  cur- 
rent will  increase  at  first  as  the  positive  potential  on  the  anode  is  increased,  and  for  each 
voltage  V,  there  will  be  a  corresponding  value  of  is  according  to  equation  (2) .  When  is 

1  It  is  evident  that  equation  (3)  may  be  used  as  a  method  for  the  determination  of  elm.  The  results  obtained,  there- 
fore, serve  to  confirm  once  more  the  conclusion  that  the  negative  current  from  the  hot  cathode  is  due  to  electrons. 


66 


has  attained  the  value  i  which  corresponds  to  saturation  thermionic  current  from  the 
filament  at  the  given  temperature,  further  increase  in  voltage  has  no  effect. 

On  the  other  hand,  with  a  given  voltage  drop,  the  current  increases  with  the  tem- 
perature until  i  is  equal  to  is,  and  further  increase  in  temperature  leads  to  no  cor- 
responding increase  in  thermionic  current.  This  is  the  case  illustrated  in  Fig.  2. 

The  existence  of  this  space  charge  effect  is  evidence  of  the  absence  of  any  positive 
ionization,  and  serves,  therefore,  as  additional  confirmation  of  the  conclusion  that  the 
currents  obtained  from  a  hot  cathode  in  a  very  high  vacuum  are  due  to  a  pure  electron 
emission,  and  are  not  dependent  upon  the  presence  of  any  small  amounts  of  gas. 

In  this  respect  the  behavior  of  a  hot  filament  in  a  good  vacuum  differs  radically  from 
the  exhibited  by  a  Wehnelt  cathode.  In  the  case  of  the  latter  the  currents  obtained  are 
due  largely  to  the  presence  of  positive  ions,  as  is  shown  by  the  absence  of  space  charge 
effects.  The  result  is  that  the  cathode  disintegrates  under  the  action  of  positive  ion 
bombardment,  and  a  rectifier  containing  such  a  cathode,  therefore,  cannot  be  used  with 
potentials  higher  than  a  few  hundred  volts  at  most.  On  the  other  hand,  in  the  case  of  a 
rectifier  containing  a  hot  filament  as 
cathode  and  exhausted  to  as  high  a  degree 
of  vacuum  as  possible,  there  is  no  con- 
duction except  by  electrons.  In  order  to 
distinguish  the  latter  type  of  hot  cathode 
rectifier  from  other  forms  in  which  posi- 
tive ions  play  an  essential  role,  the  desig- 
nation, kenotron,  has  been  specially 
coined.  This  word  is  derived  from  the 
Greek  adjective  kenos,  meaning  "empty  " 
and  the  suffix  iron  signifying  an  instru- 
ment or  appliance.  The  applicability  of 


the  name  is  self-evident. 

Having  indicated  the  possibility  of 
the  construction  of  a  high  voltage  hot 
cathode  rectifier,  we  shall  now  proceed  to 
discuss  the  principles  underlying  the  de- 
signing of  such  rectifiers. 

PRINCIPLES  OF  DESIGN  OF  KENOTRONS 
The  question  as  to  the  proper  design 
of  a  kenotron  may  be  treated  under  three 
headings : 

1.  The    amount    of    current    to    be 
rectified. 

2.  The  maximum  permissible  voltage 
loss  in  the  rectifier. 

3.  The  proper  form  of  electrodes  to  prevent  electrostatic  strains  on  the  filament. 

(1)  CURRENT  CARRYING  CAPACITY  OF  THE  KENOTRON 

The  current  carrying  capacity  of  a  kenotron  when  given  sufficiently  high  voltage 
between  the  electrodes,  is  limited  only  by  the  area  of  the  surface  emitting  electrons 
(that  is,  length  and  diameter  of  filament)  and  its  temperature.  The  data  given  in 
Table  I  and  the  curve  shown  in  Fig.  1  are,  therefore,  of  fundamental  importance  in 
this  connection.  The  next  consideration  is,  of  course,  the  "lite"1  of  the  filament  at 

1  The  "life"  of  a  filament  is  usually  taken  in  this  Iab9ratory  as  the  time  required  to  evaporate  10  per  cent  of  the 
diameter.  For  data  on  the  rate  of  evaporation  of  tungsten  filaments  the  reader  is  referred  to  the  paper  by  Langmuir,  Phys. 
Rev.,  2.  329  (1913). 

67 


Fig.  2. 


2400 


The  Effect  of  Space  Charge  on  the 
Thermionic  Currents 


2500 


any  temperature,  and  normally  the  maximum  temperature  at  which  the  filament  is 
maintained  should  be  such  that  the  "life"  of  the  filament  is  over  1000  hours  at  least. 

Thus,  a  5-mil  filament  at  2400  deg.  K.  (corresponding  to  1  watt  per  candle)  has  a 
life  of  about  4000  hours.  The  electron  emission  per  1  cm.  length  of  5-mil  filament  at 
this  temperature,  as  calculated  from  Table  I  is  15  milliamperes.  The  energy  required 
to  maintain  the  filament  at  this  temperature  is  about  4.5  watts  per  cm.  length. 

Where  the  kenotron  is  required  for  currents  of  100  milliamperes  or  more,  it  is  better 
to  use  a  7  or  10-mil  filament.  This  is  of  advantage  in  two  respects.  Not  only  is  there 
an  increase  in  area  per  unit  length,  but  also  the  life  is  much  longer  at  the  same  tempera- 
ture. On  the  other  hand,  the  temperature  can  be  increased  and  the  life  of  the  filament 
still  be  maintained  at  over  1000  hours.  Thus,  in  the  case  of  a  10-mil  filament  at  2500 
deg.  K.,  the  life  is  pretty  nearly  3000  hours,  while  the  electron  emission  is  70  milli- 
amperes per  cm.  length. 

The  data  shown  in  Table  II  are  of  great  interest  in  this  connection.  As  "safe"  tem- 
perature we  consider  that  at  which  the  life  of  the  filament  is  over  2000  hours.  The  last 
column  also  gives  the  watts  per  cm.  length  of  filament,  a  figure  which  is  of  importance 
in  calculating  the  losses  in  the  rectifier  itself.1 

TABLE  II 


Lham.  ot  Filament  in  Mils 

Safe      1  emperature 

Cm.  Length 

Watts  per  Cm.  Length'- 

5 

2475 

• 
30                                            3.1 

7 

2500 

50 

4.6 

10 

2550 

100 

7.2 

15 

2575 

200 

11.3 

(2)  VOLTAGE  DROP  IN  KENOTRON 

Owing  to  the  existence  of  the  space  charge  effect  it  is  evident  that  for  any  given  cur- 
rent carrying  capacity  i  of  a  kenotron  there  will  exist  a  voltage  drop  V  in  the  rectifier 
itself  and  the  relation  between  these  will  be  of  the  form  indicated  in  equation  (2). 

We  can  now  consider  the  manner  in  which  the  kenotron  operates  when  placed  in 
series  with  a  resistance  across  a  source  of  high  voltage. 

Let  E  denote  the  value  of  this  voltage  at  any  instant,  and  is  the  current  rectified. 
If  V  denotes  the  voltage  drop  through  the  kenotron,  and  /?,  the  resistance  of  the  load,  it 
follows  from  equation  (2)  that 

is  =  kV$  =  k  (£-«,/?)?  (2a) 

With  constant  value  of  E,  the  current  rectified  increases  as  R  is  decreased  until  i 
has  attained  the  value  i  corresponding  to  saturation  thermionic  current  at  the  temper- 
ature at  which  the  cathode  is  .maintained.  If  now  R  is  decreased  still  further,  i  remains 
constant,  and  consequently  the  voltage  over  the  kenotron  increases  beyond  that  given  by 
equation  (2).  That  is,  this  equation  gives  the  minimum  voltage  drop  through  the  keno- 
tron when  rectifying  a  given  current  is\  but  when  operating  is  series  with  a  resistance, 
the  voltage  drop  in  the  kenotron  is  that  available  above  the  isR  drop  in  load.  In  case  of  a 
short-circuit  on  the  latter,  where  R  decreases  indefinitely,  the  total  voltage  of  the  source 
is  taken  up  by  the  kenotron,  thus  liberating  the  whole  of  the  energy,  Ei,  as  heat  at  the 
anode,  and  the  latter  may  be  raised  to  a  temperature  at  which  it  will  melt  or  volatilize 
and  ruin  the  tube. 


1  The  current  necessary  to  heat  the  filament  varies  from  2  to  10  amperes,  according  to  the  diameter  of  the  filament 
and  the  temperature,  and  may  be  obtained  either  from  a  storage  battery  or  small  transformer. 

2  These  figures  are  based  upon  data  published  by  Langmuir,  Phys.  Rev.,  3.',,  401  (19KJ). 

68 


It  is  necessary  to  emphasize  this  characteristic  behavior  of  the  kenotron,  and  in 
practice  care  should  be  taken  to  provide  against  short-circuiting  of  the  load,  or  some 
form  of  protective  device  should  be  used. 

The  watts  lost  in  the  kenotron  owing  to  the  space  charge  effect  is 

WR=Vi  =  kV$  («) 

Because  of  the  high  degree  of  vacuum,  none  of  the  electrons  lose  energy  by  collision 
with  gas  molecules.  The  whole  of  their  kinetic  energy  is,  therefore,  liberated  as  heat 
at  the  anode,  just  as  the  energy  of  rifle  bullets  traveling  through  a.  comparatively 
frictionless  medium  is  converted  into  heat  at  the  target.  Denoting  the  number  of 
electrons  emitted  per  unit  area  and  per  unit  time  by  n,  and  their  velocity  by  v,  it  follows 
that  the  energy  converted  into  heat  at  the  anode  is 

WR  =  n  (y#n  v2)  =  n  e  V  =  i  V  (Ga) 

If  to  this  be  added  the  watts  WH  used  in  heating  the  filament,  then  the  total  loss  in 
energy  becomes 

WL  =  WH  +  WR  (7) 

Of  this  energy  loss,  the  whole  of  WR  and  a  large  fraction  of  WH  are  used  up  in  heat- 
ing the  anode. 

It  is  evident  that  if  the  anode  becomes  too  hot  the  rectification  will  tend  to  become 
imperfect.  The  rectifier  must,  therefore,  be  so  designed  that  the  space  charge  voltage 
is  not  great  enough  to  cause  heating  of  the  anode  when  the  requisite  current  is  being 
carried  by  the  tube.  The  amount  of  energy  (in  watts  per  square  centimeter)  required 
to  maintain  tungsten  at  a  temperature  T  is  given  by  the  equation.1 


Ws=  12.54 f    T 

\17Q3j 


(8) 


Table  III  gives  the  values  of  W$  for  different  temperatures.     The  last  column 
gives  the  corresponding  values  of  the  electron  emission  per  unit  area  in  milliamperes. 


TABLE  III 


T 

W  s 

i 

1000 

0.96 

1.2X10-11 

1500 

6.9 

6  X10-4 

1800 

16.4 

0.3 

2000 

26.9 

4.2 

2500 

77.5 

890 

From  these  data  it  may  be  concluded  that  about  10  watts  per  sq.  cm.  of  anode  area 
is  quite  permissible.  This  would  correspond  to  a  temperature  of  about  1600  deg.  K., 
that  is  a  very  bright  red  heat.  At  this  temperature  the  electron  emission  is  still  less 
than  0.02  milli-ampere  per  sq.  cm. 

(3)  ELECTRODE  DESIGN 

There  remains  only  one  other  point  to  consider  in  the  design  of  kenotrons  and 
that  is  the  prevention  of  electrostatic  strains  on  the  filament.  As  is  well  known,  the 
electrostatic  force  between  two  charged  surfaces  increases  as  the  square  of  the  voltage 
difference.  At  voltages  of  25,000  and  over,  this  force  becomes  quite  appreciable  and 
unless  special  precautions  are  taken  in  the  design  of  electrodes,  it  is  possible  at  such 
voltages  to  actually  pull  the  heated  filament  over  towards  the  anode.  When  the  keno- 

1  I.  Langmuir,  Phys.  Rev.,  34,  401  (1912).     The  same  equation  is  also  approximately  true  for  molybdenum. 

69 


tron  is  used  in  series  with  a  load  on  a  high  tension  alternating  current  circuit,  there  is  a 
very  low  potential  difference  between  the  electrode  during  the  half  cycle  that  rectifica- 
tion occurs,  while  during  the  other  half  cycle  the  whole  of  the  voltage  drop  generated 
by  the  transformer  or  other  source  of  alternating  current  occurs  in  the  rectifier  itself. 
It  is,  therefore,  necessary  to  design  the  kenotron  so  that  the  electrostatic  forces  acting 
on  the  filament  are  reduced  to  a  minimum. 

Various  types  of  construction  have  been  adopted  to  take  care  of  this  difficulty.  A 
straight  filament  in  the  axis  of  a  cylindrical  anode;  a  V-  or  W-shaped  filament  placed 
symmetrically  between  two  parallel  plates;  or  a  headlight  filament  inside  a  molyb- 
denum cap;  each  of  these  types  of  construction  has  been  found  practicable  up  to 
certain  voltages. 

Of  course,  electrostatic  forces  can  be  overcome  by  placing  the  filament  at  quite  a 
distance  from  the  anode  and  shielding  the  former  in  the  same  manner  as  is  done  by 
Coolidge  in  his  Roentgen  ray  tube.  But  under  these  conditions  the  "space  charge" 
voltage  (which  increases  with  the  first  or  second  power  of  the  distance,  see  equations 
4  and  5)  becomes  excessively  high  and  the  energy  loss  in  such  a  rectifier  would  be  alto- 
gether too  large. 

DIFFERENT  TYPES  OF  KENOTRONS 

The  different  types  of  kenotrons  mentioned  in  the  previous  section  are  illustrated 
in  Figs.  3,  4  and  5.  In  the  following  section  it  is  intended  to  discuss  briefly  the  character- 
istics of  rectifiers  that  have  been  constructed  along  these  lines  and  to  point  out  the 
relative  advantages  and  disadvantages  of  each  type. 

Fig.  3  shows  a  molybdenum  cylinder  A  with  a  coaxial  filament  F.  For  direct  cur- 
rent voltages  up  to  15,000  the  diameter  of  the  cylinder  need  not  exceed  one-half  inch 
(1.27  cm.),  while  the  length  may  be  made  as  much  as  four  inches  (10  cm.)  A.  10-mil 
filament  is  used  as  cathode. 

At  a  temperature  of  2550  deg.  K.  (see  Table  II)  the  maximum  current  obtainable 
from  such  a  kenotron  is  about  400  milli-amperes,  and  the  voltage  drop  necessary  to 
produce  this  current  as  calculated  from  equation  (4),  and  actually  observed,  is 


/400      1.27     10~3V 
\14.6X    2    X10    / 


=  145. 
The  space  charge  equation  for  this  kenotron  is 

is  =  230  X  10~3  X  Ft  milli-amperes. 

At  145  volts,  VFfl  =  145X0. 400  =  58  watts.  Also  wH  =  72  (Table  II).  The  total 
energy  used  up  in  the  rectifier  is,  therefore,  130  watts.  As  the  radiating  area  of  anode 
surface  is  about  80  sq.  cm.,  this  energy  loss  corresponds  to  slightly  over  1.5  watts  per 
sq.  cm.,  which  is  just  sufficient  to  maintain  the  anode  at  a  dull  red  heat  (1 100  deg.  K.) . 
Since  the  kenotron  is  capable  of  rectifying  0.400X15,000  =  6  kw.,  the  energy  loss  in 
the  tube  corresponds  to  about  2  per  cent  of  the  total  amount  of  energy  rectified. 

For  direct  current  voltages  up  to  75,000  or  100,000,  the  diameter  of  the  cylinder  is 
increased  to  about  5  cm.  For  mechanical  reasons  it  has  been  found  necessary,  in  this 
case,  to  attach  the  filament  to  a  molybdenum  rod  framework,  which  serves  to  increase 
the  space  charge  voltage  above  that  calculated  from  equation  (4) .  In  a  tube  intended 
to  rectify  10  kw.  at  100,000  volts  the  current  carrying  capacity  required  is  100  milli- 
amperes.  This  electron  emission  is  easily  obtained  from  about  4  cm.  of  7-mil  filament 
at  a  temperature  around  2400  deg.  K. 

70 


The  space  charge  data  of  Table  IV  were  obtained  with  one  kenotron  (No.  72)  of 
this  type  : 

These  observations  are  in  accord  with  the  equation 


The  energy  loss  in  the  tube  owing  to  this  space  charge  voltage  amounts  to  65  watts 
for  100  milli-amperes.  Adding  to  this  about  50  watts  consumed  by  the  filament,  the 
total  energy  lost  in  the  kenotron  is  about  125  watts  which  represents  only  1.25  per  cent 
of  the  total  energy  which  the  tube  is  capable  of  rectifying. 


Fig.  3.     Kenotron  Containing  Cylindrical  Anode 


Fig.  4.     Molybedenum  Cap  Type  of  Kenotron 


Fig.  5.     Kenotron  with  Filament  Between  Two  Parallel  Plates 

A  form  of  kenotron  which  is  suitable  for  voltages  not  over  10,000  and  currents 
ranging  up  to  100  milli-amperes  is  that  shown  in  Fig.  4.  It  consists  of  a  small  filament, 
such  as  is  used  in  automobile  headlights  inserted  in  a  molybdenum  cap  about  1.6  cm. 
(%  inch)  in  diameter. 

TABLE  IV 


V 

*S 

310 

33  milli-amperes 

260 

25 

130 

9 

The  following  table  gives  the  currents  actually  obtained  with  different  voltages  in 
the  case  of  kenotrons  containing  a  7-mil  headlight  filament  (No.  50),  and  5-mil  head- 
light respectively  (No.  51). 


TABLE  V 


KENOTRON  NO.  50 


KENOTRON  NO.  51 


V 

tg 

V 

<s 

20 

2.7 

100 

19 

40                  7.6                 150 

36 

80 

25.0 

200 

55 

120 

46.0 

160 

70.0 

200 

96.0 

240 

115.0 

In  the  case  of  No.  50,  the  observations  are  very  accurately  represented  by  the 
equation 


While  in  that  of  No.  51  ,  the  corresponding  equation  is 


In  neither  case  is  it  necessary  to  heat  the  filament  to  a  temperature  above  2400  deg. 
K.  The  radiating  surface  of  the  anode  is  about  4  sq.  cm.  and  the  total  energy  loss  for 
100  milli-amperes  is  about  50  watts. 

The  case  of  a  V-shaped  filament  between  two  tungsten  plates  is  illustrated  in  Fig.  5. 

In  one  case  (kenotron  No.  66)  the  plates  were  about  2  cm.  apart,  while  in  another 
(kenotron  No.  70)  the  plates  were  twice  as  far  apart.  Table  VI  gives  the  characteristics 
for  each  kenotron. 

TABLE  VI 


KENOTRON  NO.  66 

KENOTRON  NO.  70 

Fil.  Temp. 

V 

»' 

Fil.  Temp. 

V 

i. 

2340 

260 

25 

2320 

340 

28 

2370 

260 

35 

2370 

340 

50 

2410 

260 

90 

2400 

340 

54 

2450 

260 

100 

2500 

340 

60 

2500 

260 

100 

2500 

260 

42 

2500 

130 

35 

2500 

130 

14 

fs=24X10-3XT7! 

f5=9.9X10-3XFi 

The  filament  in  kenotron  No.  70  was  about  7,  while  that  in  No.  66  was  about  6  cm. 
long.1  Each  tungsten  plate  was  about  2.5  X5  cm.;  so  that  the  total  radiating  surface 
was  about  25  sq.  cm.  Kenotron  No.  66  could  be  used  up  to  about  40,000  volts, 
while  No.  70  showed  no  sparking  or  straining  of  filament  up  to  60,000  volts. 

By  using  a  W-shaped  7-mil  filament  (total  length  about  20  cm.)  between  two  tung- 
sten plates  5  cm.  square  and  situated  1  .25  cm.  apart  (kenotron  No.  54),  the  space  charge 
voltage  for  given  current  carrying  capacity  was  considerably  reduced.  The  space 
charge  equation  for  this  kenotron  was  found  to  be 


Owing  to  the  small  distance  between  the  pflates,  the  filament  was  not  situated  ex- 
actly symmetrically  with  respect  to  them,  and  it  was,  therefore,  not  thought  advisable 
to  use  the  kenotron  with  direct  current  voltages  higher  than  25,000. 

1  Owing  to  lead  losses  only  the  central  portion  of  the  filament  was  at  the  temperature-indicated. 

72 


Here  again,  the  energy  loss  in  the  kenotron  for  a  10-kw.  unit  (current  carrying 
capacity  of  400  milli-amperes)  is  well  below  2  per  cent. 

A  comparison  of  the  different  types  of  kenotrons  illustrated  above  leads  to  the  fol- 
lowing conclusions: 

(1)  For  current  carrying  capacities  up  to  500  milli-amperes,  either  a  cylindrical 
anode  with  a  filament  down  the  axis,  or  a  W-shaped  filament  placed  between  two 
parallel  plates  may  be  used.      The  first  named  type  can  apparently  be  made  much 
more  efficient  as  regards  losses  due  to  space  charge  effect. 

(2)  Where  currents  of  the  order  of  100  milli-amperes  or  less  have  to  be  rectified,  and 
the  maximum  direct  current  voltage  is  not  over  15,000,  the  molybdenum  cap  type  is 
one  that  is  simpler  mechanically  and  also  quite  efficient. 

(3)  For  voltages  up  to  100,000,  the  cylindrical  anode  type  has  proven  itself  to  be 
very  practicable  and  efficient. 

OSCILLOGRAMS  OF   PERFORMANCE  OF  KENOTRONS  WITH  ALTERNATING  CURRENT 

VOLTAGES 

In  order  to  illustrate  the  characteristics  of  a  kenotron  when  used  with  a-c.  sources, 
a  number  of  oscillograms  were  taken.  Film  Fig.  6  was  obtained  with  kenotron  No. 
54  placed  directly  across  the  60-cycle,  122- volt  terminals.  The  upper  curve  represents 
the  voltage  of  the  generator,  while  the  lower  curve  gives  the  current  through  the 


Fig.  6.     Half-wave    Rectification,  Upper    Curve    Gives  Fig.  7.     Same  as  Fig.  6.     Note  the  Effect  of 

Voltage   Over  Kenotron;   Lower   Curve  Gives  Temperature    Limitation 

Current  Rectified.     Note  the  Effect  of 
Voltage  Limitation 

kenotron.  The  effective  a-c.  voltage  was  122  and  the  maximum  voltage  180.  The 
lower  curve  shows  that  the  rectification  obtained  was  absolutely  perfect;  also  the 
peaked  nature  of  the  current  wave  shows  that  it  was  limited  by  space  charge  through- 
out the  whole  cycle. 

It  will  be  remembered  that  for  this  kenotron  the  space  charge  equation  as  obtained 
from  direct  current  measurement,  was 


It  was,  therefore,  expected  that  this  relation  ought  to  hold  quantitatively  for 
simultaneous  values  of  voltage  and  current  as  measured  on  the  oscillogram.  The  re- 
sults obtained  confirm  this  expectation  splendidly. 

The  following  table  gives  the  values  of  is  as  observed  and  calculated  for  values  of  V 
corresponding  to  different  intervals  of  a  second  t  after  the  beginning  of  the  cycle  : 


73 


Fig.  8.     Full  Rectification,  Using  Arrangement  Shown 

in  Fig.  10.     Upper  Curve — Voltage  Over  Primary 

of    Transformer;     Middle    Curve — Voltage 

Overload ;     Lower    Curve  —  Current 

Rectified.    The  Latter  was  Limited 

by    Temperature    of    Cathode 

in  Each  Case 


Fig.  9.     Same  as  Fig.  8.     The  Current  Rectified  was 

Limited  on  One  Half  Cycle  by  Tempeiature  and 

on  the  Other  Half  by  Voltage 


TABLE  VII 


V 

1 

(upper  curve) 

(lower  curve) 

(calculated) 

0.0015                                       64 

55 

53 

0.0022 

130 

150 

153 

0.0031 

165 

207 

212 

0.0042 

180 

250 

251 

A  direct  current  milli-ammeter  in  series  with  the  oscillograph  read  68  m.a. 

Film  Fig.  7  was  obtained  with  the  same  arrangement  of  apparatus,  but  the  filament 
temperature  was  made  so  low  that  the  maximum  current  obtainable  was  well  below  the 
space  charge  current  for  180  volts.  The  current  curve  begins  to  flatten  at  a  point  for 
which  V  =  90.  The  corresponding  space  charge  current  as  calculated  from  the  above 
equation  is  88  milli-amperes,  while  the  oscillogram  indicates  74  milli-amperes.  The 
direct  current  milli-ammeter  showed  a  current  of  28  m.a. 

The  oscillograms  shown  in  films  Figs.  8  and  9  were  obtained  with  an  arrangement  of 
apparatus  similar  to  that  shown  in  Fig.  10.  The  low  tension  side  of  a  potential  trans- 
former TT,  ratio  of  coils  20  to  1,  was  connected  to  the  122-volt  alternating  current 
generator,  while  the  high  tension  coils  were  connected  to  two  kenotrons  AF  and  ArFf 
as  shown  in  the  diagram.  (The  condenser  C  shown  in  the  diagram  was  omitted.) 
The  direct  current  was  taken  from  the  middle  point  of  the  transformer  and  the  fila- 
ments. A  load  of  two  250-volt  carbon  lamps  (60-watt  type)  was  connected  in  series 
with  a  milli-ammeter  and  the  current  strip  of  the  oscillograph  to  the  terminals  BBf . 
The  kenotrons  used  were  not  of  the  same  construction,  with  the  result  that  the  space 
charge  voltages  for  the  same  current  were  quite  different. 

The  upper  curve  in  each  film  gives  the  voltage  over  the  primary  of  the  transformer, 
the  middle  curve  gives  the  voltage  over  BB1 ,  while  the  lower  curve  gives  the  current 
through  the  load.  In  taking  film  Fig.  8,  the  temperature  of  the  filaments  was  main- 
tained very  low,  with  the  result  that  both  current  and  voltage  waves  were  flattened  con- 


74 


siderably.  The  slight  irregularity  in  the  amplitudes  of  the  two  half  cycles  was  due  to 
the  fact  that  it  was  almost  impossible  to  adjust  the  temperatures  of  the  two  filaments 
so  that  they  would  possess  the  same  electron  emission.  The  direct  current  milli- 
ammeter  read  100  milli-amperes. 

Film  Fig.  9  shows  an  interesting  case  in  which  the  thermionic  current  from  one  keno- 
tron was  limited  by  space  charge,  while  that  from  the  other  was  limited  by  tempera- 
ture. The  d-c.  ammeter  indicated  140  m.a.  When  taking  the  oscillogram  of  the  current 
through  the  load,  the  voltmeter  strip  was  opened,  and  when  photographing  the  wave 
of  voltage  over  load,  the  current  indicating  strip  of  the  oscillograph  was  short-circuited. 

SUMMARY 

Summarizing  briefly  what  has  been  stated  regarding  the  hot  cathode  rectifier  (keno- 
tron) it  has  been  shown  that : 

(1)  The  current  rectification  is  due  to  the  emission  of  electrons  from  a  heated  fila- 
ment in  as  good  a  vacuum  as  can  be  obtained.    The  current  carrying  capacity  of  the 
kenotron  depends  only  upon  the  area  and  temperature  of  the  filament,  and  increases 

with  the  latter  according  to  an  equation  of  the  form : 

_j> 

i  =  aVT*T  (1) 

where  i  denotes  the  saturation  thermionic  current. 

(2)  The  voltage  drop  in  the  kenotron  depends  upon  the  area,  shape  and  distance 
apart  of  the  electrodes,  and  increases  with  the  current  actually  rectified  according  to 
an  equation  of  the  form 

is  =  k.V*  (2) 

where  is  denotes  the  space  charge  current. 

When  i  is  measured  in  milli-amperes,  the  magnitude  of  k  varies  in  ordinary  cases 
from  5X10~3  for  very  high  voltage  kenotrons,  to  250X10"3  for  lower  voltage  keno- 
trons.   In  other  words,  for  a  potential 
drop  in  the  kenotron  of  100  volts,  the 
rectified   current  varies    from  5  to  250 
milli-amperes. 

As  has  been  mentioned  on  page  160, 
equation  (2)  gives  the  minimum  voltage 
drop  over  the  kenotron  when  it  is  oper- 
ated in  series  with  a  resistance  under 
most  efficient  conditions.  Owing,  how- 
ever, to  the  fact  that  the  filament  tem- 
perature limits  the  maximum  current 
which  the  kenotron  can  rectify,  it  is 
possible  for  the  voltage  over  the  latter 
to  exceed  the  value  given  by  equation  (2) 
as  the  rectifier  takes  the  difference  be- 
tween the  maximum  voltage  available  and  that  consumed  in  the  load.  Care  should,  there- 
fore, be  taken  in  using  the  kenotron  to  avoid  short  circuits  of  the  load;  or  some  form  of 
protective  device  should  be  used. 

(3)  The  actual  energy  losses  in  the  kenotron  may  be  reduced  to  less  than  two  per 
cent  of  the  total  energy  rectified  when  the  tube  is  operated  to  its  full  voltage  limit. 

(4)  Up  to  the  present,  kenotrons  have  been  constructed  for  direct  current  voltages 
as  high  as  100,000;  but  there  is  every  expectation  of  being  able  to  extend  the  field  of 
application  to  150,000  and  even  200,000  volts. 


Fig.  10.     Arrangement  for  Rectifying  Both  Half-waves, 
Using  Middle  Point  Connection  on  Transformer 


75 


F 

FI- 


U 


0 


The  maximum  current  rectified  has  been  as  much  as  1500  milli-amperes  (1.5  am- 
peres) ;  but  it  is  much  more  convenient  to  construct  these  rectifiers  in  the  form  of  1  ()-k\v. 
units  where  the  voltages  required  exceed  25,000.  For  lower  voltages,  smaller  units  are 
advisable. 

(5)  A  great  advantage  possessed  by  the  kenotron  is  that  two  or  more  of  them  can  be 
operated  in  parallel.  From  the  remarks  made  above  in  connection  with  equation  (2-a), 
it  is  evident  that  when  a  number  of  kenotrons  connected  in  parallel  are  placed  in  series 
with  a  resistance,  the  current  through  the  latter  will  control  the  voltage  drop  and  cur- 
rent through  each  kenotron  so  that  in  each  case  an  equation  of  the  form  (2-a)  is 
satisfied. 

The  kenotron  thus  possesses  at  least  two  advantages  over  the  mercury  arc  rectifier; 

firstly,  because  it  may  be  operated  at  higher  volt- 
ages, and  secondly  in  the  fact  that  several  keno- 
trons can  be  operated  in  parallel. 

APPLICATIONS  OF  THE  KENOTRON 
No  doubt  a  number  of  applications  of  this  de- 
vice will  suggest  themselves  to  electrical  engineers 
and  physicists.  A  few  words,  indicating  the  possible 
fields  of  application  that  have  already  been  sug- 
gested, will  probably  not  be  out  of  place. 

In  the  physical  laboratory  where  small  direct  cur- 
rents of  a  few  milli-amperes  at  very  high  voltages  are 
required,  as  for  spectroscopic  work,  operating  small 
discharge  tubes,  etc.,  the  kenotron  ought  to  prove 
exceptionally  useful.  An  arrangement  similar  to 
that  shown  in  Fig.  10,  and  consisting  of  two  keno- 
trons of  the  headlight  filament  type  with  a  00:1 
potential  transformer  will  act  as  a  satisfactory 
source  of  direct  current  voltages  up  to  4500  or  5000. 
By  inserting  a  condenser  C  of  sufficiently  high 

capacity  between  the  terminals  BE1  the  direct  current  obtained  may  be  made  as  free 
from  pulsations  as  desired.  The  kenotron  could  also  be  used  for  testing  the  dielectric 
strength  of  insulation  with  high  voltage  direct  currents. 

The  writer  has  obtained  as  much  as  400  milli-amperes  direct  current  at  6000  to 
7000  volts  by  using  in  the  same  manner  a  500-cycle  generator  and  a  100  to  10,000-volt 
transformer.1  By  inserting  capacity  between  the  high  tension  direct  current  terminals, 
it  was  found  possible  to  reduce  fluctuations  in  the  resulting  direct  current  to  less  than 
five  per  cent  when  100  milli-amperes  was  being  used  at  6000  volts. 

Fig.  1 1  shows  an  arrangement  of  four  kenotrons  in  which  the  whole  of  the  voltage 
generated  by  the  transformer  is  utilized.  BB1  are  the  direct  current  leads. 

The  combination  of  kenotrons  and  transformer  could  be  used  to  replace  the  cum- 
bersome static  machines  and  the  still  more  complicated  mechanical  rectifiers  that  are 
at  present  used  to  produce  high  voltage  direct  current  for  'X-ray  tubes  and  the  precip- 
itation of  dust,  smoke,  etc. 

Another  field  of  application  that  appears  to  be  very  much  within  the  limits  of  possi- 
bility is  that  of  high  voltage  direct  current  transmission.  While  this  system  has  not 
been  used  to  any  extent  in  this  country,  it  is  a  well  known  fact  that  the  Thury  system 


3 


Fig.  11.     Arrangement  of  Four  Kenotrons  for 
Making  Use  of  Full  Voltage  of  Transformer 


'The  kenotron  operates  just  as  satisfactorily  on  100,000  cycles  as  on  ordinary  frequencies. 


has  met  with  great  success  in  Europe.1     To  transmit  1000  kw.  by  100  kenotrons,  work- 
ing in  parallel  at  a  voltage  of  50,000  to  75,000  is  quite  a  feasible  proposition. 

In  conclusion  the  writer  wishes  to  express  his  indebtedness  to  Dr.  Langmuir  and 
Mr.  W.  C.  White  of  the  Research  Laboratory  for  valuable  suggestions  and  kind  co- 
operation during  the  work  on  the  development  of  the  above  device. 


1  J.  S.  Highfield,  Journ.  lust.  Elec.  Eng..  London.  3S,  471;  .',0,  848;  ,il,  640.     In  these  papers  the  advantages  of  high 
voltage  d'rect  current  transmission  are  discussed  very  fully. 


77 


A  MODEL  X-RAY  DARK-ROOM* 

BY  WHEELER  P.  DAVEY 

Anyone  visiting  the  offices  of  a  large  number  of  X-ray  practitioners  is  impressed  by 
the  small  number  of  first-class  dark-rooms.  Men  enjoying  a  large  practice  as  X-ray 
specialists  have  had  occasion  to  fit  up  rooms  very  well  adapted  to  the  purpose,  but  those 
with  smaller  practice,  or  those  who  use  X-rays  as  an  aid  to  diagnosis  in  their  own  gen- 
eral practice  do  not  seem  to  have  been  so  fortunate.  It  is  with  the  hope  of  aiding  such 
men  in  planning  an  efficient  and  convenient  dark-room,  that  this  article  is  written. 

In  planning  a  dark-room  for  X-ray  work,  space  must  be  provided  for  the  following : 

(1)  Stock  solution  of  developer;  (2)  Developing  shelf;  (3)  Sink;  (4)  Hypo  bath;  (5) 
Wash-tank;  (6)  Drying  rack;  (7)  Racks  for  holding  envelopes;  (8)  Interval  timer;  (9) 
Ventilating  fan;  and  (10)  Necessary  lights  and  switches. 

In  addition  to  the  above  it  is  usually  desirable  to  provide  room  for  a  moderate 
supply  of  plates  and  for  a  stock  of  chemicals.  A  small  viewing  screen  in  the  dark-room 
is  a  great  convenience  but  not  a  necessity.  If  frames  are  used  for  fixing,  washing  and 
drying  the  plates  (and  the  author  believes  that  they  should  be),  then  space  should  be 
provided  for  them  on  the  walls  of  the  room. 

The  room  should  be  planned  so  as  to  make  everything  as  compact  as  possible,  and 
so  that  it  is  never  necessary  to  allow  a  plate,  wet  with  hypo,  to  drip  on  the  floor.  The  two 
dark-rooms  described  in  this  article  have  been  found  satisfactory  enough  to  warrant 
them  being  called  "Model"  dark-rooms.  One  is  in  the  Research  Laboratory  of  the 
General  Electric  Company.  Space  was  at  a  premium  and  the  room  was  built  out  like  a 
closet  in  one  of  the  rooms.  The  other  is  in  the  office  of  a  well-known  surgeon  whose 
X-ray  practice  is  large  enough  to  require  the  constant  services  of  a  trained  Roent- 
genologist.  In  this  case  compactness  was  not  as  essential  as  the  ability  to  handle  a 
large  number  of  plates  quickly. 

Both  dark-rooms  were  planned  with  the  idea  of  developing  plates  entirely  by  time. 
With  the  Coolidge  tube  the  penetration  and  exposure  can  be  made  so  definite  that  this 
method  is  by  far  to  be  preferred.  Tank  development  was  considered,  but  was  thought 
not  to  be  economical  enough  of  developer  because  of  the  large  size  of  plates  used  in 
stomach  and  chest  work.  It  is  possible,  however,  to  combine  the  convenience  of  the  tank- 
method  with  the  economy  of  the  tray-method  if  the  following  technique  is  followed. 

( 1 )  Fill  the  tray  quarter-full  of  fresh  developer  at  some  standard  temperature  (say 
65  deg.  F.). 

(2)  Insert  plate. 

(3)  Shake  tray  vigorously  from  end  to  end  and  from  side  to  side  to  remove  air 
bubbles. 

(4)  Cover  tray  with  a  light-tight  cover  and  leave  undistrubed  for  ten  minutes,  as 
shown  by  the  interval-timer.    (Adjust  the  exposure  so  that  this  is  the  proper  time  of 
development.) 

(5)  After  using,  pour  developer  at  once  into  an  air-tight  bottle  just  large  enough  to 
hold  it,  and  put  in  water  bath  to  keep  cool.    If  care  is  taken  not  to  use  stale,  discolored 
developer,  this  method  has  been  found  to  be  quite  as  satisfactory  as  tank  development. 

*  Copyright,  1915,  by  General  Electric  Reriew. 

79 


•//////'""/. 


ton*; 
—Hf- 


fordtretoper 


/S' 


Shelf 


i 


Roc*. 
Fig.  1.     Ground  Plan,  Dark  Room  No.  1 


Treadles 


Fig.  2.     Back  Wall,  Dark  Room  No.  1 


Shelf  for  clock  0r 
interval  timer. 

|I 

y 

Ractc  for 

Developing 

\ 

5. 

'  '  Thermofneter 

Rack  for 

Loaaf-Ltntttat'awer 
forX-Rai)  Plates 

36" 

Fig.  3.     Right  Wall,  Dark  Room  Mo.  1 


I 

^      Cupboard  for  Crtfmt'ca/r 

y 

'•/         9 

I 

A 

_.                              .          ,-v; 

i 
D 

formulae           fy 

x 

/ 
y 

/ 

I 

i 

«~  J 

t 

TanK         6 

X 

_.          j 

X 

1           1 

Fig.  4.     Left  Wall,  Dark  Room  No.  1 


Fig.  5  Fig.  5a 

Method  of  Hanging  Curtains  in  Doorway  of  Dark  Room  No.  1 


80 


tent/toting 


Fig.  6.     Roof  Plan,  Dark  Room  No.  1 


Dry/rtq  ffac* 


Fig.  7.     Side  View  of  Hypo  and  Wash  Tanks  and 
Drying  Racks,  Dark  Room  No.  2 


12' 


O 


Fig.  8.     Top  View  of  Hypo  and  WashfTanks, 
Dark  Room  No.  2 


Fig.  9.     End  View  of  Drying  Rack,  Dark  Room  No.  2 


Developing  laUe 


Fig.  10.     Ground  Plan,  Dark  Room  No.  2 


-  drying  plate 


DryinqK'acf: 


lamps 


Jf 

Developing  Table 


J 


Treadle  for         Treadle  for    Treadle  for 
V/ew/nq  Screen  fed  light       White  light 

Fig.lt.     Wall  of  Dark  Room  No.  2 


81 


As  soon  as  the  plate  comes  from  the  developer  it  is  put  in  a  frame,  washed  in  the 
sink  and  at  once  put  in  the  hypo  tank.  After  fixing  it  is  washed  in  the  wash-tank  and 
hung  on  the  rack  to  dry.  The  use  of  the  frames  will  be  found  to  keep  the  gelatin  from 
being  marred  by  finger  prints.' 

A  glance  at  the  plan  of  either  dark-room  will  show  that  ( 1 )  a  plate  is  never  brought 
near  the  developer  after  having  once  left  it;  (2)  lights  are  turned  on  and  off  from  the 
floor, — there  are  no  switches  covered  with  hypo  to  spread  hypo-dust  into  the  developer; 
(3)  the  sink  is  between  the  hypo  and  the  developer  so  that  in  passing  from  one  to  the 
other  the  hands  may  be  washed;  (4)  the  running  water  of  the  wash-tank  is  in  contact 
with  the  hypo  tank,  thus  insuring  cold  hypo;  (5)  a  plate  can  never  drip  hypo  on  the 
floor,  even  when  being  viewed  on  the  viewing  screen. 

In  planning  the  viewing  screen,  a  great  deal  of  experimental  work  was  done  to  find 
the  best  possible  source  of  illumination  for  negatives.  Every  style  of  incandescent 
lamp  known  has  been  tried  and  compared  with  north-sky  and  with  the  mercury  arc. 

As  a  result,  it  was  found  that  the  "Blue  Photographic  Mazda"  is  by  far  the  best 
source  of  illumination  for  viewing  X-ray  negatives.  The  viewing  screens  were,  there- 
fore, designed  to  be  used  with  these  lamps. 

Plans  of  the  smaller  of  the  two  dark-rooms  are  given  in  Figs.  1  to  6.  This  room  was 
designed  for  use  with  plates  not  to  exceed  10  by  12  inches  in  size.  There  is  no  reason 
why  it  could  not  have  been  designed  for  use  with  plates  of  any  size.  The  walls  were 
built  of  dry  matched  sheeting.  The  developing  shelf  was  covered  with  lead  partly  to 
make  it  water-proof  and  partly  to  better  protect  the  plates  in  the  drawer  from  X-rays. 
Care  must  be  taken  to  have  plenty  of  overlapping  of  lead  so  that  the  rays  may  find  no 
open  crack  through  which  to  enter.  The  rheostat  (Fig.  2)  is  a  300-ohm  rheostat,  and 
has  all  the  range  needed  for  dimming  a  50-watt  red  light.  The  use  of  light  during 
development  is,  however,  not  as  necessary  with  "time  development"  as  with  the  or- 
dinary method.  It  is  the  practice  of  the  author  to  dim  the  lamp  as  much  as  possible, 
using  it  merely  as  a  point  of  reference  in  locating  things  in  the  room. 

Attention  is  called  to  the  use  of  curtains  in  the  doorways  as  a  means  of  economizing 
space.  Two  curtains  are  hung  on  separate  rods  as  shown  in  Fig.  o.  The  outside  edges 
are  fastened  permanently  to  the  doorway.  The  inside  edges  are  fastened  to  sticks 
about  one  inch  square.  These  sticks  act  as  weights  and  prevent  the  curtains  from 
blowing  in  and  causing  light-leaks.  Each  curtain  is  wide  enough  to  stretch  completely 
across  the  doorway.  A  suitable  housing  (see  Fig.  5a)  painted  black  on  the  inside  pre- 
vents light-leaking  over  the  top  of  the  curtains.  The  curtains  are  made  of  double 
thickness  of  galatea. 

Plans  of  the  larger  dark-room  are  shown  in  Figs.  7  to  11.  Attention  is  called  to  the 
method  of  drying  the  plates  with  the  fan.  The  arrangement  of  the  red  lights  is  also 
worthy  of  notice.  Instead  of  using  a  rheostat,  two  10-watt  red  lamps  were  connected  in 
series.  This  made  them  both  burn  dimly.  In  accordance  with  the  suggestion  made 
above,  they  are  used,  not  as  a  source  of  light  for  watching  the  development  of  the 
plates,  but  merely  as  marks  to  guide  the  operator  in  his  movements  about  the  room. 
One  lamp  is  placed  at  the  extreme  end  of  the  developing  table,  the  other  is  placed  over 
the  sink.  The  dimensions  of  the  hypo  and  wash  tanks  and  of  the  drying  rack  are  given 
in  Figs.  7  to  9  in  some  detail  as  a  guide  to  any  who  care  to  have  similar  work  done.  In 
Fig.  11  the  switches  governing  the  various  lamps  are  not  shown.  The  treadles  will, 
however,  indicate  where  these  switches  should  be  placed. 


82 


A  SUMMARY  OF  PHYSICAL  INVESTIGATION  WORK  IN  PROGRESS  ON 

TUBES  AND  ACCESSORIES* 

.      BY  W.   D.   COOLIDGE 

The  following  is  a  resume  of  some  of  the  X-ray  investigation  work  which  is  in  prog- 
ress in  our  laboratory. 

RADIOGRAPHIC  WORK  REQUIRING  THE  SHARPEST  POSSIBLE  DEFINITION 

Conditions  to  be  Met  in  the  Tube — For  this  purpose,  the  first  and  most  important 
desideratum  in  a  tube  is  that  it  shall  give  the  greatest  possible  X-ray  intensify  from  a 
given  sized  small  focal  spot,  for  the  use  of  a  small  focal  spot  imposes  upon  the  tube  a 
correspondingly  small  energy-input  limitation,  and  this  extends  the  time  during  which 
immobility  of  the  part  to  be  radiographed  must  be  maintained. 

To  get  the  largest  allowable  energy  input  from  a  given  sized  focal  spot  the  following 
conditions  must  be  met : 

(a)  The  distribution  of  energy  over  the  focal  spot  must  be  uniform. 

(b)  Heat  must  be  removed  from  the  focal  spot  as  rapidly  as  possible. 

The  Cathode — So  far  as  (a)  is  concerned,  the  design  of  the  cathode  is  all-important. 
Its  effect  is  best  studied  experimentally  by  making  X-ray  pinhole  camera  pictures  of  the 
focal  spot  with  various  types  of  cathode. 

A  convenient  method  of  making  such  pinhole  camera  focal-spot  pictures  is  as  fol- 
lows :  Take  a  square  sheet  of  lead,  say  a  sixteenth  of  an  inch  thick  and  of  the  right  size 
to  take  the  place  usually  occupied  by  the  diaphragm  in  the  tube  stand.  Make  a  conical 
depression  in  the  center  with  a  machinist's  prick-punch,  or  other  suitable  tool.  With  a 
knife  remove  the  tiny  prominence  produced  in  this  way  on  the  back  of  the  lead  plate, 
and  then,  with  a  pin  or  small  drill  open  the  hole  to  the  desired  size,  say  0.02  inches  in 
diameter.  (If  the  hole  is  too  large  the  focal  spot  picture  will  lack  sharpness,  and  if  it  is 
too  small,  the  required  time  of  exposure  will  be  needlessly  long.)  Put  the  lead  plate  and 
the  tube  in  place  in  the  holder  and  lay  the  photographic  plate  on  a  table  below.  The 
size  of  the  focal  spot  picture  can  be  varied  at  will  by  raising  or  lowering  the  tube  holder. 
If  it  is  to  be  of  natural  size,  the  distance  from  plate  to  pin-hole  should  be  equal  to 
that  from  pin-hole  to  focal  spot.  If  it  is  desired  to  study  the  focal  spot  in  greater  detail, 
it  is  well  to  make  the  former  distance  twice  the  latter,  which  will  give  a  twofold  mag- 
nification. 

It  is  a  good  plan  to  make  two  exposures,  one  of  which  is  much  longer  than  the  other. 
The  long  exposure  will  then  show  the  full  extent  of  the  focal  spot,  and  the  shorter  one 
will  show  the  distribution  of  energy  over  it.  (In  the  case  of  gas-filled  tubes,  one  should 
use  the  same  current  and  voltage  as  are  to  be  employed  in  regular  work,  for  with  this 
type  of  tube  the  size  of  focal  spot  varies  greatly  with  the  vacuum,  being  smaller  the 
harder  the  tube  is.  With  a  hot-cathode  tube,  on  the  other  hand,  the  size  of  focal  spot  is 
always  the  same  regardless  of  the  current  and  voltage  employed.) 

No  conclusion  concerning  uniformity  of  distribution  of  energy  can  be  drawn  from 
the  focal  spot  picture,  unless  care  is  taken  to  guard  against  over-exposure,  for  otherwise 

*  Copyright,  1915,  by  American  Journal  of  Roentgenology. 

83 


even  a  very  bad  distribution  will  appear  uniform.  This  is  shown  by  the  upper  pair  of 
focal  spot  pictures  in  Fig.  1 .  The  right-hand  one  was  exposed  3  seconds  and  the  other 
25.  These  are  twice  natural  size. 

One  interesting  type  of  cathode  is  shown  in  Fig.  2.  The  focusing  device  is  a  hemi- 
spherical bowl,  and  the  filament  spiral,  instead  of  being  convex  outward,  is  concave, 
with  the  same  radius  of  curvature  as  the  bowl,  and  is  located  in  the  center  and  in  the 
surface  of  the  latter.  Except  for  a  small  hole  in  the  center,  this  gives  a  focal  spot  which 
shows  a  very  nearly  uniform  distribution  of  energy  and  which  is  also  very  sharp  and 
clean-cut  at  the  edges.  It  is  reproduced  (again  in  twice  natural  size)  in  Fig.  1 ,  the  lower 
pair. 

Target  Design. — In  connection  with  (b),  various  modifications  of  the  target  have 
been  tried.  The  most  obvious  experiment  was  perhaps  to  try  the  copper-backed  tung- 
sten target  such  as  is  used  in  the  gas-filled  tube,  for  this  brings  copper,  which  has  three 
times  the  heat  conductivity  of  tungsten,  close  to  the  focal  spot. 

For  a  long  time  we  were  not  able  to  make  this  experiment  with  the  hot-cathode  tube 
for  the  reason  that  we  did  not  know  how  to  exhaust  it,  to  the  required  high  degree  of 
vacuum,  with  anything  but  tungsten  or  molybdenum  electrodes.  We  finally  learned 
how  to  do  this  and  then  made  up  two  tubes,  one  with  a  copper-backed  and  the  other 
with  our  regu  ar  all-tungsten  target,  taking  care  that  the  focal  spot  should  have  the 


O 


Fig.  1 


Fig.  2 


same  size  in  both.  (This  was  subsequently  checked  by  the  X-ray  pinhole-camera  pic- 
ture method.)  The  tubes  were  run  on  a  definite  voltage  for  a  definite  time,  with  a 
definite  current.  The  focal  spot  was  then  inspected  to  make  sure, that  melting  had  not 
taken  place.  The  current  was  then  raised  successively  to  higher  values  until  the  focal 
spot  was  finally  melted,  sufficient  time  being  allowed  between  experiments  for  the  target 
to  cool. 

For  exposures  of  3  seconds  or  less,  starting  cold,  there  appeared  to  be  no  difference 
in  the  two  targets,  or  at  any  rate  not  more  than  the  experimental  error.  The  experi- 
ment is  of  course  a  little  crude,  but  not  so  much  so  as  one  might  at  first  think ;  for  it  can 
be  repeated  as  many  times  as  desired.  (This  is  made  possible  by  the  fact  that  the 
bright  shiny  area  which  has  been  melted  at  the  focal  spot  may  be  made  to  take  on  a 
dulled  or  frosted  appearance  by  operating  the  tube  even  for  as  short  a  time  as  a  minute 
with  an  energy  input  which  is  safely  below  that  sufficient  to  cause  melting.  Having 
frosted  the  surface  in  this  way,  there  is  of  course  no  trouble  in  again  finding  how  much 
it  takes,  starting  cold,  to  melt  the  focal  spot.) 

I  For  30-second  exposures  the  copper-backed  target  was  able  to  carry  about  twice  as 
much  energy  as  the  all-tungsten  target.  This  difference  was  due  merely  to  the  greater 
heat  capacity  of  the  heavy  copper-backed  target,  and  could  have  been  eliminated  by  the 
use  of  a  larger  tungsten  target. 


84 


For  still  longer  exposures  the  tables  were  turned,  and  the  advantage  was  all  with  the 
all-tungsten  target.  This  was  of  course  due  to  the  fact  that  it  is  not  permissible  to  let  a 
copper  target  get  above  a  dull  red  heat,  as  otherwise  it  volatilizes  badly  and  deposits 
on  the  bulb. 

The  experiment  showed  that  a  sufficiently  large  all-tungsten  target  would  do  any- 
thing that  could  be  done  with  the  present  standard  copper-backed  target.  (It  was 
moreover  free  from  the  above  mentioned  limitation  of  the  latter.) 

The  question  naturally  arises  as  to  why  copper  with  its  three-fold  greater  heat  con- 
ductivity does  not  assist  more  in  getting  heat  away  from  the  focal  spot.  The  reason  is 
this,  that  the  tungsten  disc  in  the  standard  copper-backed  target  is  so  thick  (about  2.5 
millimeters) .  The  only  place  where  high  heat  conductivity  is  important  is  in  the  im- 
mediate neighborhood  of  the  focal  spot  and  this  position  is  in  either  case  occupied  by 
tungsten.  Copper  could  be  made  to  help  if  the  thickness  of  the  tungsten  button  could 
safely  be  considerably  reduced.  But  if  this  is  done  the  copper  back  of  the  focal  spot 
melts.  This  melting  of  the  copper  is  accompanied  by  expansion  and  this  bulges  out 


Fig.  3 


Fig.  4 


the  tungsten  at  this  point.  Upon  cooling,  contraction  of  the  copper  takes  place  and  an 
empty  space  is  left  between  the  tungsten  and  the  copper  at  the  focal  spot.  When  the 
tube  is  again  used,  the  tungsten  at  the  focal  spot,  not  being  in  contact  with  the  copper 
behind  it,  melts  right  through,  thus  ruining  the  target. 

If  exposures  were  made  in  rapid  succession  with  a  very  fine  focus  tube,  there  would 
be  an  advantage  in  increasing  the  present  size  of  the  all-tungsten  target  so  as  to  give  it 
more  heat  capacity  and  more  radiating  surface.  The  same  increase  in  heat  capacity  and 
radiating  surface  can  be  secured  with  less  weight  by  such  a  construction  as  is 
shown  in  Fig.  3  and  again,  in  section,  in  Fig.  4.  In  this  case  the  target  consists  of  a  very 
thick  tungsten  cylinder  pressed  into  a  block  of  molybdenum.  The  latter  metal  helps 
in  this  way,  that  it  is  lighter  than  tungsten,  which  means  more  radiating  surface  for 
a  given  weight,  and  that  it  has  about  twice  the  heat  capacity  for  a  given  weight. 

But,  in  the  way  in  which  the  very  fine  focus  tube  now  seems  to  be  used  in  practice, 
the  exposure  would  always  start  with  a  relatively  cool  target.  For  this  and  the  above 
reasons  there  appears  to  be  no  advantage  in  increasing  the  heat  capacity  and  radiating 
surface  of  the  present  all-tungsten  target  in  the  fine  focus  tube. 

The  above  relations  may  be  seen  at  a  glance  by  an  inspection  of  the  following  in- 
complete table.  It  shows  current  values  just  insufficient  to  cause  melting,  starting  with 
the  target  cold  and  with  the  tube  always  backing  up  the  same  spark  gap  (4^  in.).  The 
focal  spots  were  all  of  the  same  size. 


Type  of  Target 

Milli-amperes 

3 
sec. 

10 
sec. 

30 
sec. 

Equi- 
librium 

Copper-backed  tungsten 

38 

35 
35 

30 
25 
15 

Molybdenum-backed  tungsten 

15 

12.5 

Regular  tungsten 

30 

Water  Cooling  of  the  Target. — This  will  help  the  sharp  focus  tube,  at  least  to  this 
extent,  that  the  exposure  will  then  always  start  with  a  cold  target.  It  will,  furthermore, 
make  it  possible  for  the  tube  to  carry  almost  as  much  energy  for  an  indefinite  period  as 
it  does  for  a  very  short  one.  This  subject  has  yet  to  be  quantitatively  investigated 
for  the  actual  conditions  met  in  practice. 

Direct  Current  Excitation. — Considerable  help  may  also  come  from  the  use  of  a  con- 
stant potential  continuous  current  source.  Some  preliminary  experiments  by  Dr.  Hull 
seem  to  indicate  that,  working  with  the  proper  voltages  to  give  the  same  mixture  of 
rays  (having  the  same  proportions  of  the  different  wave-lengths) ,  the  X-ray  intensity 
for  the  same  heating  of  the  target  is  about  one  third  greater  with  constant  potential 
than  it  is  with  the  usual  sine  wave.  This  gain  would  not  seem  to  iustifv  the  increased 


Fig.  5 

complications  in  such  a  generator,  but  careful  investigation  may  show  that  the  advan- 
tage is  really  much  greater  and  for  the  following  reasons :  When  a  tube  is  run  hard  on 
the  ordirary  sources  of  current  supply,  the  fecal  spot  always  gets  very  rough  and  often 
cracks.  The  sharp  points  and  edges  so  formed  are  in  a  very  poor  position  to  give  up 
their  heat  to  the  heavy  metal  mass  behind  and,  to  this  extent,  the  limiting  energy  input 
must  suffer.  The  roughening  action  appears  to  be  due  to  the  forces  of  heat  expansion 
and  contraction  brought  into  play  by  the  intermittent  character  of  the  bombardment 
(usually  60  cycles  per  second)  and  is  lacking  in  the  case  of  continuous  current  at  con- 
stant potential.  This  subject  merits  careful  quantitative  investigation. 

Rotation  of  the  Target. — As  an  entirely  different  method  of  increasing  the  allowable 
energy  input  for  a  given  size  of  focal  spot,  we  are  trying  Prof.  Elihu  Thomson's  scheme 
of  rotating  the  target.  This  rotation  leaves  the  size,  and  position  in  space,  of  the  focal 
spot  unchanged,  but  results  in  spreading  the  energy  over  a  much  larger  area.  We  have 
already  had  such  tubes  in  operation  with  the  target  running  at  750  r.p.m.,  and  with  the 
focal  spot  describing  a  circle  %  in.  in  diameter.  One  of  them  is  shown  in  Fig.  5.  The 
target  in  this  instance  was  rotated  by  means  of  an  externally  placed  permanent  horse- 
shoe magnet  acting  on  the  soft  iron  armature  within. 

The  crude  experiments  which  we  have  already  made  seem  to  indicate  that,  with 
the  above  mentioned  speed  of  750  r.p.m.,  and  with  the  focal  spot  describing  a  circle 

86 


Y\  in.  in  diameter,  we  can  carry  two  or  three  times  as  much  energy  for  the  size  of  focal 
spot  used,  as  we  can  with  the  target  stationary. 

It  seems  as  though,  to  justify  the  complications  inherent  in  the  use  of  a  rotating 
target,  we  need  something  more  like  a  10-fold  increase  over  the  stationary  target.  If 
bearing  troubles  can  be  eliminated,  this  should  be  attainable,  and  more  work  is  to  be 
done  along  this  line. 

A  HOODED  TARGET 

For  best  definition  in  radiographic  work,  the  rays  should  all  come  from  the  focal 
spot.  With  the  type  of  hot-cathode  tube  now  on  the  market,  there  is  a  very  consider- 
able emanation  of  X-rays  from  all  over  the  target.  By  means  of  the  ionization  method, 
we  have  investigated  this  quantitatively,  and  have  found  that  the  integrated  value  of 
the  rays  from  }/£  of  the  target  surface  exclusive  of  the  focal  spot  (this  is  of  course  all 
that  counts,  as  those  coming  from  the  side  which  is  turned  away  from  the  plate  do  not 
affect  it)  is  about  }/§  of  the  intensity  from  the  focal  spot. 


Fig.  6 

All  of  our  experimental  evidence  at  present  points  to  the  fact  that  these  disturbing 
X-rays  from  the  body  of  the  target  are  caused  by  secondary  cathode  rays  coming  from 
the  focal  spot  and  prevented  from  going  to  the  glass  as  they  do  in  the  ordinary  gas- 
rilled  tube,  by  a  negative  electrification  of  the  bulb. 

To  reduce  the  intensity  of  this  disturbing  factor,  a  cylindrical  cap  or  hood  of 
molybdenum  has  been  attached  to  the  front  of  the  target  (see  Fig.  2).  The  cathode 
rays  enter  this  hood  through  a  small  hole  in  the  front,  and  the  X-rays  emerge  through 
a  second  hole  in  the  side. 

The  hood  offers  the  following  advantages:  (a)  It  reduces  the  radiation  from  the 
surface  of  the  target  exclusive  of  the  focal  spot  to  about  }/§  (in  the  present  design)  of 
what  it  is  without  the  hood.  Its  effect  in  this  direction  is  well  shown  by  the  following 
experiment :  A  lead  plate  with  several  small  holes  was  placed  about  2%  inches  above  a 
photographic  plate  and  20  inches  away  from  the  focal  spot  (see  Fig.  6).  To  have  the 
effect  as  pronounced  as  possible,  no  cone  or  other  diaphragm  was  used.  Two  equally 
timed  exposures  were  made  on  the  same  plate,  one  with  a  hooded  tube  and  the  other 
with  an  unhooded  one  having  the  same  sized  focal  spot.  The  same  voltage  and  milli- 


87 


amperage  were  used  in  both  cases  (see  Fig.  7).  The  black,  sharply  defined  circles  were 
produced  by  the  rays  coming  from  the  focal  spot,  while  the  balance  of  the  picture  is  due 
to  the  rays  coming  from  the  surface  of  the  target  other  than  the  focal  spot.  The  fact 
that  the  same  intensity  of  rays  was  emitted  from  the  focal  spot  in  both  tubes  was  con- 
firmed by  the  equal  density  of  the  circular  areas  in  the  two  pictures.  It  is  clear  that  the 
halos  in  the  right-hand  picture,  made  with  the  hooded  target,  are  much  less  intense  than 
they  are  in  the  other  one. 

The  fact  that  the  intensity  of  radiation  from  the  back  of  the  target  has  been  in  this 
way  reduced  to  3^  of  its  value,  might  well  lead  the  roentgenologist  to  hope  for  much 
better  definition  in  radiograms  of  human  beings,  from  the  use  of  the  hood.  Experience 
teaches,  however,  that  the  gain  is  very  slight.  It  can  be  seen  by  the  direct  comparison 
of  two  plates,  but  could  with  difficulty  be  confirmed  without  making  the  direct  com- 
parison. The  reason  for  this  doubtless  lies  in  the  fact  that  in  those  cases  where  one 
could  possibly  hope  to  see  it,  namely,  with  thick  objects  such  as  the  head,  it  is  almost 


Fig.  7 

masked  by  the  much  more  disturbing  effect  of  the  secondary  rays  coming  from  the 
object  itself. 

(b)  The  hood  reduces  the  danger  to  the  operator  as  it,  together  with  the  cathode, 
cuts  off  all  but  the  narrow  cone  of  direct  rays  which  is  actually  desired. 

(c)  It  greatly  reduces  the  amount  of  inverse  current  which  can  pass  through  the 
tube  when  the  latter  is  run  on  alternating  current  and  with  sufficient  energy  to  heat  the 
focal  spot  to  temperatures  approximating  that  of  the  cathode  spiral. 

(d)  It  reduces  the  blackening  of  the  bulb  which  results  if  a  tube  is  run  so  hard  that 
the  focal  spot  is  melted  and  volatilized. 

(e)  It  reduces  the  danger  from  putting  excessive  currents  through  the  tube,  for  a 
melted  globule  of  tungsten,  which  might  otherwise  strike  the  glass  bulb  and  crack  it, 
is  now  very  likely  to  be  intercepted  by  the  hood. 

For  certain  purposes,  the  hood  promises  to  be  very  useful.  In  the  light  of  experi- 
ments, to  be  described  later,  however,  on  the  use  of  much  smaller  bulbs  and  on  the  use 
of  water-cooled  targets,  it  may  not  have  as  much  importance  as  would  otherwise  attach 
to  it. 


88 


SIZE  OF  BULB 

This  can  be  very  greatly  reduced,  probably  to  4  inches  for  most  work. 
The  chief  advantages  to  be  derived  from  the  adoption  of  a  smaller  bulb  are  the  fol- 


(1)  For  the  same 
amount  of  X-ray  pro- 
tection, it  permits  of 
a  great  reduction  in 
the  size  and  weight  of 
the  tube  holder. 


o 


Fig.  8 


(2)  It  permits  a 
closer  approach  of 
the  diaphragms  to  the 
focal  spot  and  hence 
gives  better  defini- 
tion. 

This 


(3)  It  permits  a  closer  approach  of  the  focal  spot  to  the  object  to  be  rayed, 
is  in  some  cases  desirable 

(4)  It  decreases  the  danger  of  tube-breakage  in  handling. 

TIPLESS  BULB 

We  find  that  the  seal-off  tip,  can  be  removed  from  the  bulb  to  the  end  of  one  of  the 
side  arms,  so  that  it  is  later  covered  by  the  terminal  cap.  This  seems  to  considerably 
simplify  the  design  of  a  satisfactory  tube  holder,  and  it  also  reduces  the  danger  of  tube 
breakage. 


Fig.  9 


HIGH  POTENTIAL  TUBE  WITH  HOODED  TARGET  AND  INTERNAL  FILTER 
At  the  Cleveland  meeting  of  the  Society  last  year,  I  reported  on  some  experimental 
work  on  tubes  which  could  be  operated  backing  up  a  15-  or  even  a  20-inch  spark.  It 
seemed  at  the  time  that  it  might  be  interesting  to  have  such  high  potentials  tried  out 
for  therapeutic  work.  Upon  studying  the  situation,  however,  it  developed  that  with 
the  very  thick  aluminum  filters  which  it  seemed  desirable  to  use,  the  problem  of  pro- 
tecting the  patient,  with  the  cross-fire  technique  and  many  ports  of  entry,  was  more 
difficult  than  anything  which  had  been  met  before.  It  could  of  course  be  taken  care  of 
by  the  development  of  a  suitable  tube  holder.  The  problem  is  greatly  simplified  by  the 
use  of  a  hooded  target  with  a  piece  of  tungsten  foil  or  other  filtering  material  covering 
the  opening  in  the  hood  (see  Fig.  8).  With  this  arrangement,  no  direct  unfiltered  rays 
can  leave  the  tube. 


89 


i— Fan 


Fig.  10 


HIGH  POWER  TUBE  WITH  WATER-COOLED  TARGET 

The  experimental  development  of  this  tube  has  had,  so  far,  for  its  objective,  con- 
tinuous service  at  very  high  power,  for  industrial  applications.  But  it  is  already  clear 
that  there  are  certain  advantages  connected  with  this  tube  which  must  be  taken  very 
seriously  by  the  roentgenologist.  The  most  striking  differences  between  it  and  the 
present  hot-cathode  tube  are  the  following :  Owing  to  the  effective  cooling  of  the  target, 
very  large  amounts  of  energy  can  be  carried,  without  heating  the  focal  spot  to  the  point 
where  inverse  current  can  pass.  The  tube  is,  therefore,  capable  of  rectifying  its  own 

current  even  for  heavy  loads.  The 
cooling  of  the  target  also  prevents  the 
strong  heating  of  the  glass,  due  to 
radiation  from  the  target,  which  takes 
place  in  the  present  tube  on  continu- 
ous operation. 

Description. — The  general  design 
may  be  seen  in  Fig.  9.  The  cathode 
is  the  same  as  has  been  used  in  the 
regular  Coolidge  tube.  The  target 
consists  of  a  copper-backed  piece  of 

tungsten  which  is  silver-soldered  to  the  end  of  a  thin- walled  copped  tube.  Inside  this 
copper  tube  is  another  smaller  one  through  which  water  is  brought  to  the  back  of  the 
target,  from  which  point  it  escapes  through  the  space  between  the  two  tubes. 

As  shown  in  the  diagram,  the  tube  is  connected  directly  to  the  terminals  of  a  high- 
tension  transformer,  without  any  auxiliary  rectifying  device. 

Performance. — A  good  idea  of  the  exceedingly  regular  operation  of 'the  water-cooled 
tube  is  given  by  the  performance  of  tube  X-179,  which  was  run  continuously  for  42 
hours,  carrying  a  current  of  100  milli-amperes  and  supporting  a  potential  of  70,000  volts 
(peak  value).  During  the  whole  test,  the  filament  current  necessary  to  make  the* tube 
draw  100  milli-amperes  varied  only  from  4.58  amperes  to  4.50  amperes.  These  values 
represent  the  extreme  change  observed  during  42  hours.  It  was,  furthermore,  a  per- 
fectly regular  progressive  change, 
taking  place  in  the  direction  of  a 
lowering  of  the  filament  current  with 
the  time  of  operation.  The  change 
was  not  only  small,  but  was  clearly 
due  not  to  any  change  in  vacuum  but 
to  a  gradual  pulling  out  of  the  cathode 
spiral  which  can  be  entirely  obviated 
by  a  suitable  cathode  design.  At  the 
end  of  the  test,  the  target  had  sprung 
a  leak.  In  the  next  design  of  target  the 
water  was  brought  closer  to  the  face.  This  one  ran  68  hours  continuously  at  100  milli- 
amperes  and  70,000  volts,  and  then  failed  just  as  the  first  one  had.  A  later  target 
design  appears  to  eliminate  the  trouble  experienced  in  the  above  mentioned  models. 

Capacity. — Our  life-tests,  as  stated  above,  have  been  run  at  100  milli-amperes  and 
70,000  volts.  But  the  same  tubes  have  been  run  continuously  at  200  milli-amperes  and 
70,000  volts  and  at  100  milli-amperes  and  100,000  volts.  There  is  no  apparent  reason 
why,  by  increasing  the  size  of  target  and  focal  spot,  the  allowable  energy  input  should 
not  be  raised  to  at  least  25  or  50  kilowatts  for  continuous  operation. 


Fig.  11.    25-kw.  G-E  Transformer.    Tube  load  :  100  M.  A.,  70  Kv. 

Upper  curve  :  tube  current.     Lower  curve:  useful  voltage 

above  zero  line  and  inverse  voltage  below 


90 


Fig.  12.    25-kw.  G-E  Transformer.    Tube  load  :   200  M.  A.,70Kv. 

Upper  curve  :  tube  current.     Lower  curve  :  useful  voltage 

above  zero  line  and  inverse  voltage  below 


Different  Water  Cooling  Systems. — Over  99  per  cent  of  the  energy  supplied  to  such 
a  tube  is  delivered  to  the  water  used  in  cooling  the  anode.  A  simple  calculation  shows 
that  an  energy  input  of  200.0  milli-amperes  at  70,000  volts  is  sufficient  to  raise  the  tem- 
perature of  a  liter  of  water  from  20  deg.  centigrade  to  the  boiling  point  in  33  seconds. 
This  shows  that,  for  continuous  operation  at  this  high  power,  it  is  necessary,  if  boiling 
is  to  be  prevented,  to  pass  through  the  target  at  least  2  liters  of  cold  water  per  minute. 
There  are  two  easy  ways  of  doing  this.  The  first  consists  in  connecting  the  tube  right  to 
the  faucet  and  using  tap  water.  To  do  this  necessitates  grounding  one  side  of  the  X-ray 
transformer.  This  appears  to  be  unobjectionable  for  work  calling  for  voltages  not  to 
exceed,  say,  70,000,  and  this  system 
when  applied  to  the  above  combination 
appears  to  make  the  simplest  conceiv- 
able high  power  X-ray  outfit.  It  seems 
admirably  adapted  to  the  industrial 
applications  at  least,  such  as  steriliza- 
tion, insecticide,  metal  radiography, 
and  the  coloring  of  glass,  porcelain,  etc. 
The  second  method  of  handling  the 
cooling  problem  involves  the  use  of -a 
radiator  (the  automobile  types  are 
admirably  suited  to  the  purpose),  a 

water-circulating  pump,  and  a  fan  (see  Fig.  10).  With  this  method  the  whole  water 
system  is  insulated,  and  the  middle  point  of  the  transformer  secondary  is  grounded. 
As  a  result,  corona  troubles  are  reduced,  and  in  so  far  as  this  effect  is  concerned,  it 
becomes  possible  to  go  to  twice  the  voltage  that  could  be  used  with  the  other  system. 
It  is,  therefore,  the  method  which  recommends  itself  for  therapeutic  work.  The 
ordinary  Ford  radiator  with  a  12  in.  desk  fan  and  a  small  water  pump  takes  care  of  at 
least  7  kw.  continuously.  By  using  a  more  powerful  fan  or  a  cellular  (instead  of  a 
tubular)  radiator,  the  effectiveness  of  this  cooling  system  could  readily  be  doubled. 
A  small  desk  fan  or  Sirocco  blower  is  also  necessary  for  cooling  the  bulb. 
Suitable  Source  of  Current  Supply. — The  water-cooled  tube,  when  rectifying  its 
own  current,  places  new  demands  on  a  transformer.  Only  one  half- wave  passes  through 

the  tube,  with  the  result  that,  unless 

the  transformer  is  suited  to  the  work, 
the  voltage  on  this  half -wave  may  be 
much  lower  than  on  the  other  half. 
This  would  introduce  a  difficulty  in 
the  measurement  of  voltage.    A  spark- 
gap,  for  example,  when  used  in  the 
ordinary    way,    would    measure     the 
inverse  voltage  and  not   that   corre- 
sponding to  the  X-rays  produced.    Furthermore,  if  the  inverse  voltage  was  much  higher 
than  that  in  the  right  direction,  there  would  be  a  needless  strain  on  the  insulation  of  the 
transformer,  and  increased  corona  from  the  wires  leading  to  the  tube. 

The  oscillograms,  F:gs.  1 1  and  12,  show  the  behavior  of  the  tube  under  heavy  load 
on  a  suitable  transformer.  The  upper  curves  show  that  the  tube  completely  rectifies  its 
own  current,  as  there  is  nothing  below  the  zero  line.  The  lower  curves,  indicating  volt- 
age, were  obtained  by  the  use  of  a  very  sensitive  vibrator  in  series  with  a  high  resistance 
water  column  which  was  connected  in  parallel  with  the  tube. 


Fig.  13.     W.  &  B.  Transformer.     Tube  load  :    100  M.  A.,  41  K 
Useful  voltage  above  zero  line  and  inverse  voltage  below 


91 


The  potential  curves  show  but  little  departure  from  a  sine  wave.  The  application  of 
a  millimeter  scale  to  the  curves  shows  that  in  the  case  of  the  100  milli- ampere  load  the 
peak  value  of  the  inverse  voltage  is  only  2.5  per  cent  greater  than  that  of  the  direct  volt- 
age, and  that  even  in  the  case  of  the  200  milli- ampere  load,  it  is  only  4  per  cent  greater. 

Fig.  13  shows  the  voltage  curve  obtained  with  the  same  tube  excited  from  one  of  the 
regular  X-ray  transformers  now  on  the  market.  (It  happened  to  be  a  Waite  &  Bartlett. 
The  mechanical  rectifier  was  not  used.)  Inspection  of  the  curve  shows  that  the  inverse 
voltage  is  much  higher  than  the  direct  voltage.  Measurement  of  the  peak  values  made 
on  the  oscillogram  shows  them  to  be  in  the  ratio  of  31.5  to  14.8;  or,  in  other  words,  the 
inverse  voltage  is  113  per  cent  higher  than  the  direct  or  useful  voltage. 

It  is  obvious  from  the  above  that  the  high  power  water-cooled  tube  can  be  used  with 
perfect  satisfaction  on  a  large  transformer  of  suitable  design,  with  no  auxiliary  rectify- 
ing device.  It  is  equally  clear  that  it  must  not  be  used  as  a  self -rectifying  tube  on  such 
transformers  as  those  now  on  the  market,  which  were  designed  with  a  view  to  utilizing 
both  half-waves 

In  these  experiments,  furthermore,  the  resistance  of  the  mains  supplying  the  trans- 
formers was  kept  very  low  and  the  voltage  was  controlled  through  the  field  excitation 
of  the  alternator.  Even  with  a  suitable  transformer,  the  scheme  does  not  lend  itself  to 
resistance  control,  for  the  effect  of  resistance  in  series  with  the  primary  is  to  lower  the 
useful  secondary  voltage  but  not  the  inverse.  In  practice  the  simplest  method  for 
controlling  voltage  would  be  by  means  of  an  auto-transformer  or  by  the  use  of  a  num- 
ber of  taps  on  the  primary. 

The  above  considerations  will  apply  with  equal  force  to  the  use  of  a  kenotron  for  the 
rectification  of  current  for  the  ordinary  Coolidge  tube.  The  combination,  to  be  satis- 
factory for  work  where  heavy  tube  currents  are  ever  to  be  employed,  necessitates  the 
use  of  a  relatively  large  transformer,  that  is  unless  some  special  method  is  employed  for 
the  reduction  of  inverse  voltage. 

ADVANTAGES   OF   WATER  COOLED  TUBE  EXCITED    DIRECTLY  FROM  TRANSFORMER 
WITHOUT  AUXILIARY  RECTIFYING  DEVICE 

They  may  be  summarized  as  follows : 

(1)  High  power  for  continuous  operation — there  is  no  power  limit  in  sight. 

(2)  Noiseless  operation. 

(3)  Simplicity  and  reliability. 

(4)  This  system  will  make  it  possible  to  go  without  difficulty  to  voltages  much 
higher  than  100,000. 

(5)  It  is  perfectly  adapted  to  the  operation  of  any  desired  number  of  tubes  running 
in  parallel. 

(6)  Owing  to  the  high  power  which  can  be  employed  and  to  the  fact  that  the  target 
is  cooled,  the  system  is  well  adapted  to  the  production  of  homogeneous  rays.    The  tar- 
get facing  can  be  made  of  any  one  of  a  large  number  of  the  metals  and  the  rays  can  then 
be  passed  through  a  filter  of  the  same  material. 

(7)  The  water-cooled  target  permits  of  the  use  of  a  much  smaller  bulb  than  would 
otherwise  be  necessary  for  the  same  power. 

(8)  The  close  voltage  regulation  which  is  necessary  for  the  satisfactory  operation  of 
the  above  system  is  also  helpful  in  the  exact  duplication  of  results,  for  a  considerable  de- 
gree of  inaccuracy  in  the  adjustment  of  filament  temperature  has  no  appreciable  ef- 
fect on  voltage,  and  hence  on  penetration. 

92 


THE  USE  OF  A  KENOTRON  FOR  RECTIFYING  CURRENT  WITH  THE  PRESENT  COOLIDGE 

TUBE 

Provided  a  suitable  transformer  is  used  a  kenotron*  may  be  substituted  for  the 
mechanical  rectifier,  for  the  operation  of  the  ordinary  Coolidge  tube. 

As  has  been  pointed  out  in  the  preceding  section,  the  transformer  requirements  are, 
for  this  purpose,  exactly  the  same  as  for  the  self -rectifying  water-cooled  tube. 

There  is  much  to  be  said  in  favor  of  the  use  of  the  kenotron,  and  the  advantages 
(2) ,  (3) ,  (4)  and  (8)  which  have  been  cited  for  the  water-cooled  tube  operated  directly 
from  a  transformer,  apply  equally  here. 

THE  CONTROL  OF  FILAMENT  TEMPERATURE 

To  simplify  and  render  more  accurate  the  technique  of  the  Coolidge  tube,  some 
source  of  filament  current  supply  which  is  much  better  than  the  storage  battery  or  the 
simple  transformer  is  needed.  The  desideratum  is  a  source  of  perfectly  constant 
potential.  This  would  allow  the  filament  temperature  to  be  controlled  by  means  of  a 
dial  switch,  each  point  of  which,  with  the  same  tube,  would  always  mean  the  same  tem- 
perature, and  hence  the  same  milliamperage. 

After  working  with  magnetos,  boosters,  Nernst  iron  wire  ballasts,  etc.,  we  have  finally 
found  what  appears  to  be  a  perfectly  satisfactory  solution  of  the  problem.  It  con- 
sists in  the  use  of  a  specially  designed  transformer  in  conjunction  with  the  usual  fila- 
ment current  transformer.  It  is  the  function  of  the  former  to  make  it  possible  to  de- 
liver to  the  filament  constant  current,  even  though  the  line  voltage  may  fluctuate 
greatly  and  suddenly.  The  regular  filament  current  transformer  is  retained  merely  to 
provide  the  necessary  insulation  between  the  filament  circuit  and  the  supply  mains. 
This  combination  draws  its  energy  directly  from  the  mains  in  the  case  of  alternating 
current  installations,  and  from  the  alternating  current  side  of  the  rotary  in  the  case  of 
direct  current  installations.  For  induction  coils  run  from  direct  current,  where  there  is 
no  alternating  current  supply,  it  will  be  necessary  to  add  to  the  above  combination  a 
small  rotary. 

The  special  constant  potential  transformer  referred  to  above  has  no  moving  parts 
and  no  time  lag.  The  present  model  allows  the  filament  current  to  fluctuate  less  than 
one  per  cent  when  the  supply  voltage  varies  25  per  cent.  This  means  that  it  com- 
pletely takes  care  of  the  ordinary  fluctuations  in  the  supply  voltage,  due  to  causes  ex- 
ternal to  the  X-ray  installations  and  of  the  sudden  drop  caused  by  the  closing  of  the 
X-ray  switch  as  well. 

Change  in  filament  temperature  may  be  effected  by  means  of  a  dial  switch  which 
controls  a  resistance  connected  in  series  with  the  primary  of  the  ordinary  filament 
current  transformer. 

The  above  method  does  away  with  the  need  of  any  accurate  adjustment  of  either  a 
resistance  or  an  impedance  and  with  the  need  of  the  accurate  reading  of  an  ammeter 
or  milliammeter.  Each  point  on  the  dial  means  always  the  same  filament  temperature 
and  the  same  milli-amperage. 

It  will  mean  a  great  simplification  in  the  use  of  the  tube.  In  those  cases,  for  exam- 
ple, where  it  is  used  for  radiographic  work  exclusively  and  where  the  operator  prefers 
to  use  the  same  penetration  and  the  same  milliamperage  for  everything,  he  will  never 
need  to  think  of  anything  but  his  main  switch  and  his  watch.  It  will  also  be  a  great 
convenience  in  those  cases  where  the  technique  of  the  operator  combines  in  the  same 
case  fluoroscopy  and  radiography.  For  the  filament  temperature  will  be  carried  from 
that  requisite  for  fluoroscopy  to  that  required  for  radiography  by  moving  the  dial 

*  Saul  Duschmann,  General  Electric  Review,  1915. 

93 


switch  from  one  button  to  another,  and  these  two  buttons  can  be  adjacent  to  one 
another. 

ACCURATE  MEASUREMENT  OF  TUBE  VOLTAGE 

The  best  method  by  far  would  appear  to  be  the  one  which  is  generally  used  by  the 
electrical  engineer  in  high  potential  work.  It  consists  in  the  use  of  a  voltmeter  and  a 
special  potential  jcoil  properly  located  in  the  transformer.  If  there  is  sufficient  iron  and 
copper  in  the  transformer,  this  method  is  very  simple  and  should  be  exceedingly  satis- 
factory. If,  on  the  other  hand,  a  small  transformer  is  made  to  do  heavy  work,  there 
will  be  a  large  correction  to  be  applied  to  the  voltmeter  reading  and  this  correction 
will  be  a  variable  quantity  dependent  on  the  load.  This  last  applies,  and  with  greater 
force,  to  the  use  of  a  voltmeter  across  the  primary.  In  either  of  the  two  latter  cases, 


Fig.  14 


however,  there  should  be  no  trouble  in  making  an  accurate  voltage  calibration,  for  the 
different  loads  actually  employed,  by  means  of  a  sphere  gap. 

This  instrument  (see  Fig.  14)  has  been  standardized  by  the  American  Institute  of 
Electrical  Engineers.  A  table  of  spark  lengths,  and  the  precautions  necessary  in  using 
the  instrument  and  the  corrections  to  be  applied,  are  given  in  their  Standardization 
Rules  (Edition  of  July  1,  1915).  For  the  voltages  now  used  in  X-ray  work,  the  62.5 
mm.  spheres  will  be  found  most  satisfactory.  The  distance  between  them  is  adjustable 
by  means  of  the  milled  nut  at  the  top,  as  shown  in  the  diagram,  and  can  be  read  on  the 
micrometer  head  connected  with  it.  The  glass  U-tubes  on  either  side  of  the  gap  are 
filled  with  distilled  water.  Their  purpose  is  to  interpose  sufficient  resistance  in  the  cir- 
cuit to  prevent  destructive  arcing  on  the  surface  of  the  spheres.  The  tubes  may  well  be 
about  %  inches  internal  diameter  and  of  such  length  that  there  shall  be  in  each  of  them 
about  15  inches  of  water  column. 

In  X-ray  work,  the  usefulness  of  the  sphere  gap  will  doubtless  be  confined  exclusively 
to  calibrating  the  combination  of  voltmeter  and  transformer. 


Research  Laboratory  of  the  General  Electric  Co.,  Schenectady. 


94 


ROENTGEN  RAYS  FROM  SOURCES  OTHER  THAN  THE  FOCAL  SPOT  IN 
TUBES  OF  THE  PURE  ELECTRON  DISCHARGE  TYPE* 

BY  W.  D.  COOLIDGE  AND  C.  N.  MOORE 

Early  in  the  history  of  the  hot  cathode  type  of  roentgen  tube,  and  before  any  other 
roentgenograms  had  been  made  with  it,  the  pinhole-camera-picture  method  was  applied 
in  a  preliminary  study  of  the  source  of  the  roentgen  rays  from  the  tube.  This  work 
showed  that  the  entire  surface  of  the  target  was  active  in  producing  roentgen  rays.  In 
spite  of  this  fact,  however,  experiments  showed  that  good  roentgenographs  of  the  vari- 
ous parts  of  the  human  body  could  be  produced  with  the  tube. 

Since  that  time,  various  workers  have  felt  that  still  better  roentgenographic  results 
might  be  obtained  if  all  rays  other  than  those  coming  from  the  focal  spot  could  be  elimi- 
nated. 


Fig.  1.     Arrangement  of  Apparatus  for  making  Pinhole-Camera 
Roentgenograms  shown  in  this  Article 

It  seemed  desirable  to  investigate  the  subject  pretty  thoroughly  to  learn,  first,  how 
the  effect  of  roentgen  rays  other  than  those  coming  from  the  focal  spot  could  best  be 
minimized  and,  second,  whether  the  elimination  of  this  effect  would  result  in  roentgeno- 
graphic images  of  greater  diagnostic  value.  The  investigation  also  seemed  worth  while 
for  the  light  which  it  would  throw  on  the  modus  operandi  of  the  hot  cathode  type  of 
roentgen  tube  and  for  the  use  to  which  such  information  could  be  put  in  the  develop- 
ment of  new  forms  of  this  tube.  The  investigation  consisted  mainly  in  a  pinhole- 
camera-picture  study  of  various  forms  of  the  tube. 

The  following  pinhole-camera  roentgenograms  were  all  made  under  the  same  general 
conditions:  The  same  diaphragm,  with  a  hole  0.020  inches  in  diameter,  was  used 
throughout;  the  distance  from  the  length  axis  of  the  tube  to  the  pinhole  was  always  the 
same,  15-in.,  and  the  distance  from  the  pinhole  to  the  photographic  plate  was  5-in. ;  and 
in  all  length-views  of  the  target  the  tube  was  so  placed  that  the  neck  of  the  target  was 
directly  opposite  the  pinhole  (see  Fig.  1).  The  plate  was  in  a  closed  box  made  of  lead 
J/g-in.  thick.  Unless  otherwise  noted,  the  same  voltage,  that  corresponding  to  a  7-in. 
spark  gap  between  points,  was  used  throughout,  and  the  same  current,  4  milli-amperes, 

*From  General  Electric  Review,  April,  1917,  pp.  272-281. 


95 


and  the  same  time,  10  minutes.  Time  development  for  4  minutes  at  the  same  definite 
temperature  and  with  fresh  developer  was  used  on  all  of  the  plates.  To  still  further 
guard  against  the  possibility  of  unequal  development,  as  well  as  to  make  comparison 
easier,  a  pinhole  camera  roentgenogram  of  the  standard  tube  has  in  many  cases  been 
put  on  the  same  plate  with  the  particular  type  which  was  being  investigated.  No 
protection  of  any  kind,  not  even  the  usual  lead  glass  bowl,  was  used  around  the  tube  and 
no  cone  or  diaphragm. 

PlNHOLE-CAMERA  ROENTGENOGRAMS  MADE  WlTH  THE  STANDARD 

HOT-CATHODE  TUBE 

Fig.  2  was  made  with  a  standard  medium  focus  tube,  operating  with  the  voltage 
corresponding  to  a  7-in.  spark  gap  between  points  and  with  4  milliamperes  of  current  for 

40  minutes.  The  image  of  the  focal 
spot  is  greatly  overexposed,  so  that  the 
roentgenogram  gives  a  very  exagger- 
ated impression  of  the  amount  of 
radiation  coming  from  the  remainder 
of  the  target.  The  overexposure  was 
intentional,  the  idea  being  to  show  the 
relative  intensities  of  the  radiation 
coming  from  those  portions  of  the 
target  other  than  the  focal  spot.  The 
roentgenogram  shows  that  the  entire 
target,  including  the  molybdenum  stem 
and  the  adjacent  end  of  the  iron  sup- 
port-tube, gives  off  roentgen  rays  and 
must,  therefore,  be  bombarded  by  cath- 
ode rays  from  some  source  or  other. 
The  circle  described  about  the  middle 


Fig.  2.     Roentgenogram  made  with  Standard 
Medium-Focus  Tube 


of  the  focal  spot  as  a  center  indicates 
the  location  of  the  glass  bulb.  As 
will  be  pointed  out  in  connection  with 
some  of  the  later  plates,  the  pro- 
duction of  roentgen  rays  from  the 
entire  surface  of  the  target  must  be 
accounted  for  by  the  reflection*  of 
cathode  rays  from  the  focal  spot. 
These  reflected  rays,  consisting  as 
they  do  of  negatively  charged  par- 
ticles, electrons,  cannot  go  to  the 
cathode,  as  it  is  negatively  charged. 
Nor  can  they  go  to  the  glass  bulb,  as 
it  is  charged  to  the  potential  of  the 
cathode.  They  come  away  from  the 
focal  spot,  as  will  be  shown  later, 
with  almost  as  high  a  velocity  as  that 
with  which  they  approached  it;  but, 
by  the  repulsion  of  the  charges  on  the  cathode  and  on  the  glass,  they  are  forced  to  go 


Fig.  3.     Roentgenograms  of  Front  and  Back  of  Target. 

Note    that,    except    for     Working    Face, 

Intensity  is  Practically  the  same 

Front  and  Back 


*  The  term  reflection  has  been  used  througout  this  paper  to  include  both  true  reflection  and  secondary  emission. 


96 


back  and  strike  the  target  again,  and  many  of  them  are  doubtless  again  reflected  and 
again  forced  to  return  to  the  target.  Roentgen  rays  must  be  produced  with  each 
collision  with  the  target.  It  is  clear  from  Fig.  2  that  relatively  few  electrons  are  able 
to  get  into  the  anode  arm,  because  of  the  repulsion  of  the  negative  charge  on  the  bulb 
back  of  the  anode  and  on  the  walls  of  the  anode  arm.  The  intensity  of  the  rays  com- 

ing  from  the  stem  will  be  seen  to  fall 

off  very  rapidly  at  the  point    where 
the  circle  crosses  the  stem. 

Those  surfaces  of  the  cathode  struc- 
ture which  face  the  anode  are  also  seen 
to  give  off  roentgen  rays.  The  origin  of 
these  is  entirely  different  in  its  nature. 
They  are  secondary  roentgen  rays  pro- 
duced by  the  primary  roentgen  rays 
coming  from  the  target. 

Fig.  3  shows  the  front  and  back  of 
a  target.  Except  for  the  working  face, 
the  intensity  is  seen  to  be  essentially 
the -same  both  front  and  back.  The 
intensity  of  radiation  appears  greater 
at  the  peripheral  than  it  does  in  the 
central  portion  of  the  picture,  and,  in 
general,  the  greater  the  angle  of  inclina- 
tion of  the  surface  with  reference  to  the 

Fig.  4.     Roentgenograms    made  with  7-in.  and  3  *4 -in.  Tubes         plane    Qf    the     photographic    plate,    the 

greater    is    the  apparent   intensity   of 

radiation.  This  is  simply  due  to  the  fact  that  the  roentgen  rays  are  emitted  with 
equal  intensity  in  all  directions  from  each  and  every  point  in  the  surface. 

In  Fig.  4  the  roentgenogram  at  the 
top  was  made  with  a  7-in.  tube  and 
that  at  the  bottom  with  a  .S^-in. 
tube.  The  circle  described  about  the 
middle  of  each  focal  spot  as  a  center 
gives  the  location  of  the  glass  bulb  in 
each  case.  The  figure  brings  out 
strikingly  the  effect  of  the  negative 
charge  on  the  glass  in  determining  the 
extent  of  the  target  to  be  bombarded 
by  the  reflected  cathode  rays.  The 
total  production  of  roentgen  rays 
outside  of  the  focal  spot  is  doubtless 

the  same  in  both  cases.  (No  attempt  has  been  made  to  confirm  this  quantitatively.) 
But  there  is  very  little  coming  from  that  portion  of  the  stem  which  is  within  the  anode 
arm  and,  therefore,  there  is,  of  necessity,  a  greater  intensity  of  radiation  from  the  body 
of  the  target  in  the  small  tube  than  there  is  in  the  large  one. 

Fig.  5  shows  the  effect  of  voltage.  Both  roentgenograms  were  made  with  the  same 
tube,  the  lower  one  with  a  2-in.  spark  gap  (between  points)  and  the  upper  one  with  a 
10-in.  spark  gap.  The  exposures  were  so  chosen,  by  means  of  a  preliminary  set  of  ex- 
periments, that  the  photographic  effect  of  the  rays  frcm  the  focal  spot  should  be  the 


Fig.  5.     Roentgenograms  showing  the  effect  of  voltage. 
Upper,  10-in.  Spark  Gap;  Lower,  2-in.  Spark  Gap 


same  in  both  cases.  This  meant  an  exposure  of  30000  milliampere  seconds  for  the  2-in. 
spark  gap  and  3500  for  the  10-in.  spark  gap.  The  body  of  the  target,  which  is  of  tung- 
sten, shows  a  somewhat  different  distribution  of  radiation  in  the  two  cases,  the  working 
face  and  the  entire  forward  portion  showing  less  radiation  at  the  2-in.  spark  gap,  and 
the  rear  portion  more.  The  molybdenum  stem  radiates  very  feebly  at  the  2-in.  spark 
gap-,  so  that  there  is  no  trouble  is  seeing  where  it  is  attached  to  the  tungsten  head  of  the 
target.  The  explanation  for  the  feebler  radiation  of  molybdenum  is,  as  will  be  seen 
later,  to  be  found  in  its  lower  atomic  weight.  At  the  10-in.  spark  gap  the  molybdenum 
stem  is  seen  to  radiate  even  more  strongly  than  the  tungsten.  This  is  due  to  the  fact 
that,  at  this  higher  voltage,  there  is  added  to  the  general  radiation  of  the  molybdenum 
the  characteristic  radiation  of  that  element. 

To  give  an  idea  of  the  relative  intensity  and  penetrating  power  of  the  radiation  from 
the  focal  spot  and  from  the  balance  of  the  target,  two  roentgenograms  of  a  Benoist 
penetrometer  were  made,  one  with  the  rays  from  the  front  and  the  other  with  the  nys 
from  the  back  of  the  target.  These  are  shown  in  Fig.  6.  Both  were  made  with  a  2-in 
spark  gap.  The  one  to  the  right  was  made  by  rays  coming  from  the  front  of  the  target 
and  was  given,  as  measured  in  milliampere  seconds,  1/10  the  exposure  of  the  other. 
The  central  circular  areas  show  very  nearly  the  same  photographic  action,  indicating 
that  the  total. radiation  from  the  back  of  the  target  is,  photographically  measured, 
about  '1/10  of  that  from  the  front.  This  means  that  in  the  ordinary  roentgenogram, 
made  from  the  front  of  the  target,  there  will  be,  photographically  measured,  about  1/9 
as  much  radiation  from  the  body  and  stem  of  the  target  as  there  is  from  the  focal  spot. 
This  fraction  would,  in  ordinary  work,  be  reduced  a  little  by  the  use  of  a  diaphragm 
and  cone.  The  penetration  is  seen  to  be  about  4*/£  Benoist  from  the  back  and  5  Benoist 
from  the  front. 

With  a  10-in.  spark  gap,  as  shown  by  Fig.  7,  the  conditions  are  seen  to  be  much  the 
same.  As  before,  the  right-hand  roentgenogram  was  made  from  the  front,  and  the 
other  from  the  back  of  the  target.  The  exposure  from  the  front,  as  measured  in  milli- 
ampere seconds,  was  1/9  of  that  from  the  back  and  was  apparently  a  little  too  much. 
The  factor  of  1/10  used  in  the  preceding  figure  would  have  been  better.  The 
penetration  from  the  front  was  9  Benoist,  and  from  the  back  8  Benoist. 

Photographically  measured  then,  there  appears  to  be  from  the  front  of  the  target, 
over  this  wide  range  of  voltage,  approximately  1/9  as  much  radiation  coming  from  the 
balance  of  the  surface  as  there  is  from  the  focal  spot,  and  of  but  slightly  less  penetrating 
power. 

METHODS  FOR  MINIMIZING  THE  EFFECT 
Hooded  Target 

One  of  the  different  methods  which  has  suggested  itself  for  reducing  the  effect  of  the 
radiation  from  outside  of  the  focal  spot  is  the  use  of  a  metal  hood  attached  to  the  work- 
ing end  of  the  target.  This  is  illustrated  in  Fig.  8  and  has  been  described  in  an  earlier 
paper.*  The  cathode  rays  enter  through  a  hole  in  the  end  of  the  hood  and  the  roentgen 
rays  emerge  through  a  hole  in  the  side.  Fig.  9  shows  the  roentgenogram  from  such  a 
hooded  target,  and  above  it,  for  comparison,  that  of  the  target  in  a  standard  tube.  Fig. 
10  shows  the  same  thing  except  that  the  targets  are  turned  side  wise.  The  position  of 
the  focal  spot  has,  in  the  case  of  the  hooded  target,  been  indicated  by  those  roentgen 
rays  from  the  focal  spot  which  penetrated  the  molybdenum  hood,  which  in  this  case  was 
only  yj  in.  thick.  A  comparison  of  the  two  roetgenograms  shows  that  the  amount  of 
radiation  from  the  body  and  stem  of  the  target  is  greatly  reduced  by  the  use  of  the  hood. 

*  W.  D.  Coolidge,  Am.  Journal  of  Roentgenology,  Vol.  II,  p.  882  (1915). 

98 


(Quantitative  measurements  with  the  ionization  chamber  show  that  with  the  design  of 
hood  used  in  this  experiment,  the  reduction  is  to  about  1/6  of  what  it  would  be  with- 
out the  hood.) 

The  roentgenograms  of  the  hooded  target  are,  of  themselves,  sufficient  to  prove  that 
at  least  all  of  that  radiation  from  outside  the  focal  spot  which  is  eliminated  by  the  use 
of  a  hood  must  be  due  to  cathode 
rays  reflected  from  the  focal  spot. 
Without  such  proof,  one  might  think 
that  much  of  this  might  be  due  to 
failure  on  the  part  of  the  cathode 

foCUSing  device  tO  bring  tO    the    focal  Fig.  ».     Metal  Hood  on  Working  End  of  Target 

spot   all    of   the   primary  bundle   of 

cathode  rays.     An  inspection  of  the  hooded  target  roentgenogram  in  Fig.  10  shows 

the  true  explanation.  Through  the  hole  in  the  side  of  the  hood  can  be  seen  a  small 

portion  of  the  inner  surface  of  the 
hood,  and  this  area  shows  a  very 
vigorous  roentgen  ray  production. 
The  hood  is  in  good  metallic  contact 
with  the  target,  and  is,  therefore,  of 
necessity  at  the  same  potential  as 
the  latter.  There  is  then  no  force 
to  deflect  any  of  the  rapidly  moving 
cathode  ray  stream  out  of  its  course 
and  to  bring  it  to  the  inner  surface 
of  the  hood.  The  only  possible 
explanation  remaining  then,  is  that 
the  vigorous  bombardment  of  the 
inner  surface  of  the  hood,  which 
manifests  itself  not  only  through 
roentgen  ray  production  but  also  by 

a  very  rapid  heating  of  the  hood  (upon  starting  to  excite  the  tube  with  a  cold  target 

the  hood  comes  to  incandescence  before  the  body  of  the  target  does),  is  due  to  cathode 

rays  reflected  from  the  focal  spot. 

The  reflection  is  not  prevented  by 

the  hood,  but  the  roentgen  rays 

resulting  from  the  reflected  cathode 

rays  are  produced  on  the  inner  sur- 
face of  the  hood  instead  of  being 

produced  on  the  outer  surface  of 

the    target.     The    fact    that    the 

roentgenogram  shows  more  in- 
tensity on  the  lower  side  of  the 

target  is  easily  accounted  for  by 

assuming  that  a  few  of  the  reflected 

rays  come  out  through  the  hole  in 

the   side   of   the   hood   and   then 

bombard  the   body   and   stem   of  the   target   on   that   same   side. 

Fig.  11  shows  two  hooded  targets  differing  only  in  this  respect,  that  in  the  lower  one 
a  piece  of  thin  tungsten  foil  (0.0008  in.  thick)  has  been  bound  over  the  hole  in  the  side  of 


Fig.  6.     Roentgenograms  of  a  Benoist  Penetrometer.     Right, 
Rays  from   Front  of    Target;    Left,    Rays    from   Back 
of  Target.    Exposure  for  Front  of  Target  1/10  that 
for  Back  of  Target.     2-in.   Spark  Gap 


Fig.  7.     Roentgenograms   for  Front  and  Back  of  Target  as 
in  Fig.  6,  but  with  10-in.  Spark  Gap.     Exposure  1  to  9 


99 


the  hood.    This  would  necessarily  prevent  any  of  the  reflected  cathode  rays  from  escap- 
ing at  this  point.    An  inspection  of  the  roentgenogram  shows  that  the  use  of  the  foil 

has  prevented  the  difference  in  in- 
tensity on  the  two  sides  of  the  target 
which  is  noticeable  in  the  case  of  the 
hooded  target  without  the  foil.  It 
seems  most  probable,  especially  in  the 
light  of  Fig.  14,  which  will  be  de- 
scribed later,  that  the  small  amount 
of  radiation  remaining  on  the  surface 
and  stem  in  the  case  of  the  hooded 
target  with  the  foil  is  due  to  reflected 
cathode  rays  coming  from  the  focal 
spot  and  emerging  through  the  hole  in 
the  end  of  the  hood,  through  which 
the  primary  cathode  rays  enter.  The 
number  of  electrons  escaping  in  this 
wav  would  naturallv  be  relativelv 


Fig.  9.     Upper,  Roentgenogram   of  Hooded  Target; 

Lower,  Roentgenogram  of  Target  of 

Standard  Tube 


small,  owing  to  the  fact  that  the  angle 
subtended  by  the  hole  is  small,  and  to 
the  further  fact  that  these  electrons 
have  to  move  out  against  the  electro- 
static repulsion  of  the  cathode. 

Cathode  Placed  Very  Close  to  Anode 

Another  method  which  has  pre- 
sented itself  for  reducing  the  intensity 
of  the  roentgen  radiation  from  the 
body  and  stem  of  the  target  consists 
in  bringing  the  cathode  so  close  to  the 
anode  that  the  reflected  cathode  rays 
are  driven  almost  straight  back  to  the 
focal  spot  by  the  electrostatic  repul- 
sion of  the  cathode. 

In  Fig.    12  the  front  end  of  the 


Fig.  11.     Upper,  Roentgenogram  of  Hooded  Target; 

Lower,  Roentgenogram  of  Hooded  Target 

with  Tungsten  Foil 


molybdenum  focusing  tube  of  the  cath- 
ode was  only  2.5  mm.  from  the  anode 
at  its  nearest  point.  (This  front  end 
of  the  focusing  tube  is  clearly  visible 
in  the  figure,  due  to  the  secondary 
roentgen  rays  emitted  by  it.)  The 


Fig.  10.     Same  as  Fig.  9,  except  that  Targets 
are  turned  Sidewise 


Fig.  12.      Roentgenogram  of  Target  of  Tube  in  which 
Cathode  is  placed  Very  Close  to  Anode 


100 


Fig.   13.     Tube  in  which  Working  Face  of   Target  is  at  Right 

Angles  to  Axis  of  Tube.     Distance  between  focusing 

device  and  face  of  anode,  2.3  millimeters 


roentgenogram  shows  that  the  reflected  electrons  which  started  out  in  this  direction 
were  forced,  by  electrostatic  repulsion,  to  again  bombard  the  target  on  its  face,  close 
to  the  focal  spot.  Those  leaving  in  other  directions,  where  the  distance  between  the 
end  of  the  cathode  and  the  inclined  face  of  the  anode  was  greater,  were  able  to  get  much 
further  away  from  the  focal  spot. 

Fig.  13  is  the  diagram  of  a  tube  representing  a  more  radical  experiment.  In  this 
tube  the  working  face  of  the  target 
was  at  right  angles  to  the  axis  of  the 
tube  and  the  distance  between  the  end 
of  the  focusing  device  and  the  face  of 
the  anode  was  only  2.3  mm.  (In  this 
tube,  the  filament  spiral,  to  prevent 
its  being  pulled  out  under  the  strong 
electrostatic  attraction,  was  set  much 
further  back  than  usual  in  the  focus- 
ing device.) 

At  the  bottom  of  Fig.  14  is  seen  the  roentgenogram  of  this  tube,  the  rays  being  taken 
from  the  face  of  the  target  in  a  direction  making  a  very  small  angle  with  the  face. 

The  small  round  spot  in  the  center  of 
the  target  face  is  the  focal  spot,  seen 
through  the  molybdenum  focusing 
tube.  The  concentric  circular  ring 
around  this  is  due  to  the  reflected 
cathode  rays  which,  instead  of  being 
allowed  to  bombard  the  target  all 
over  its  surface,  have  been  forced 
to  return  to  points  close  to  the  focal 
spot.  Comparison  of  this  roentgen- 
ogram with  that  of  the  standard  tube  above  it  shows  how  very  effective  this 
method  has  been. 

Influence  of  the  Atomic  Number  of  the  Metal  on  Which  the  Reflected  Cathode  Rays  Fall 

Fig.  15  is  the  roengtenogram  of  a 
target  consisting  of  a  tungsten  button, 
19  mm.  in  diameter,  set  in  a  block  of 
copper  32  mm.  in  diameter,  the  latter 
being  supported  by  a  stem  of  molyb- 
denum screwed  into  it.  The  line 
between  the  tungsten  and  the  copper 
is  quite  marked  in  the  roentgenogram, 
and  the  roentgen  ray  production  is 
clearly  less  from  the  copper  than  it  is 
from  the  tungsten.  This  is  in  accord- 
ance with  data  published  by  Kaye  on 
the  efficiency  of  different  metals  in 
producing  general  roentgen  radiation 
(not  including  characteristic)  under  a 

given  cathode  ray  bombardment.  His  measurements  were  made  with  25,000  volts 
on  the  tube. 


Fig.  15.     Roentgenogram  of  Target  Consisting  of 
Tungsten  Button  set  in  Block  of  Copper 


Fig.  14.     Lower,  Roentgenogram  of  Target  of  Tube  shown 

in  Fig.  13;  Upper,  Roentgenogram  of  Target 

of  Standard  Tube 


101 


INDEPENDENT       RADIATION 


Metal 

Atomic  Weight 

Intensity  of  Radiation 

Platinum 
Tungsten.     .    .          

195.2 
184.0 

100 
91 

Molybdenum 

96.0 

51 

Copper.  . 

63.6 

33 

Aluminum  

27. 

10 

Fig.  16.     Tube   in  which  Target  is  placrd  at  Right 
Angles  to  Length-axis  of  Tube 


He  found,  as  the  table  shows,  that 
the  efficiency  decreases  as  the  atomic 
weight  decreases,  and  is  approxi- 
mately proportional  to  it.  By  caloriz- 
ing  the  copper  on  the  surface,  or  by 
otherwise  covering  it  with  aluminum, 
or  by  replacing  the  copper  with  mag- 
nesium or  some  metal  of  still  lower 
atomic  weight,  it  would  be  possible 
to  still  further  reduce  the  roentgen- 
ray  production  from  the  surface  of 
the  target  outside  of  the  focal  spot. 
The  method  is  different  in  principle 
from  either  of  the  preceding  in  that 
it  involves  no  attempt  to  reduce  the 

intensity  of  bombardment  of  the  surface  by  the  reflected  cathode  rays,  but  rather 

reduces  the  efficiency  of  the  roentgen- 

ray   production   resulting   from    this 

bombardment . 


Target  Placed  at  Right  Angles  to  the 

Length-axis  of  the  Tube 

In  this  method  no  attempt  is  made 
to  influence  the  production  of  roent- 
gen  rays  on  the  surface  of  the  target. 
The  target  is  merely  inserted  in  the  side 
of  the  bulb  as  shown  in  Fig.  16,  instead 
of  in  the  usual  position,  and  the  rays  are 
then  taken  from  the  end  instead  of  the 
side  of  the  target.    As  a  result,  the  rays 
from  the  body  and  stem  of  the  target  do 
not  affect  the  roentgenogram,  as  they  are 


Fig.  18.     Tube  having  Diaphragm  Parallel  to 
Target  and  Insulated  from  it 


Fig.  17.     Right,  Roentgenogram  of  Target  of  Tube  shown  in 
Fig.  16;  Left,  Broadside  View  of  Target  for  Comparison 


Fig.  19.     Two  Roentgenograms  of  Target 
of  Tube  shown  in  Fig.  18 


102 


intercepted  by  the  face.  This  is  shown  in  the  roentgenogram  (Fig.  17),  in  which  the 
broadside  view  of  the  target  is  also  given  for  comparison.  In  this  simple  form  this 
method  would  be  very  effective  for  work  calling  for  a  narrow  angle  of  rays ;  it  would 
not  be  effective  when  the  angle  is  wide  enough  so  that  the  face  of  the  target  no  longer 
intercepts  the  rays  coming  from  the  body  and  stem. 

Diaphragm  Within  the  Tube,  Parallel  to  the  Target  and  Insulated  from  It 

This  construction  is  illustrated  in  Fig.  18.  The  diaphragm  is  made  of  some  metal 
opaque  to  the  roentgen  rays,  such  as  molybdenum  or  tungsten.  It  may  be  attached  to 
the  cathode,  or  it  may  be  supported  by  the  glass  of  the  anode  arm.  In  either  case  the 
diaphragm  will  be  at  cathode  potential  when  the  tube  is  operating,  and  hence  will  not 
be  bombarded  by  the  reflected  cathode  beam.  Therefore,  it  will  not  of  itself  be  a  source 
of  roentgen  rays.  Fig.  19  shows  two  roentgenograms  of  such  a  target.  The  lower  one 
is  a  face  view.  The  diaphragm  itself  does  not  show  at  all  in  this  view,  indicating  that  it 
undergoes  no  cathode  ray  bombardment.  The  body  and  stem  of  the  target  show 
through  the  diaphragm  a  little,  owing  to  the  fact  that  it  consisted  of  molybdenum  only 
0.010  in.  thick.  The  upper  roentgenogram  was  made  after  rotating  the  tube  through 
an  angle  of  about  120  degrees.  It  shows  that  from  the  inner  surface  of  the  diaphragm 
there  is  a  small  amount  of  roentgen  radiation,  but  this  is  seen  to  take  place  only  from 
that  portion  of  the  end  of  the  diaphragm  which  is  in  the  path  of  the  roentgen  rays  from 
the  focal  spot,  and  is  hence  secondary  roentgen  radiation.  The  roentgenogram  shows 
that  the  molybdenum  piece  not  only  serves  as  a  diaphragm,  but  that,  by  being  at 
cathode  potential  as  it  is,  it  causes  some  of  the  reflected  cathode  ray  bombardment  to 
be  transferred  by  electrostatic  repulsion  from  the  front  to  the  back  of  the  anode. 

Limitations  of  the  Various  Methods 

Each  of  these  methods,  as  applied  to  the  present  standard  tube,  interferes  in  some 
way  with  its  general  usefulness.  The 
hood,  to  be  very  effective,  should 
have  a  small  window  for  the  emer- 
gence of  the  roentgen  rays,  and  this 
limits  the  size  of  plate  which  can  be 
covered  with  the  tube.  The  same 
consideration  applies,  although  with 
less  force,  to  the  internal  diaphragm 
which  is  insulated  from  the  anode. 

The  placing  of  the  target  at  right 
angles  to  the  length  axis  of  the  tube 
makes  the  tube  awkward  to  handle. 
In  the  simple  form,  it  is,  furthermore, 
effective  in  only  the  narrow  central 
cone  of  rays.  The  tube  can,  of  course, 
be  used  to  cover  a  wide  angle,  but  it 

is  only  the  central  portion  of  the  resulting  roentgenograph  which  can  profit  appreciably 
by  the  method. 

The  placing  of  the  cathode  very  close  to  the  anode  (as  close  as  2.5  mm.,  for  example) 
limits  the  potential  which  can  be  used  on  the  tube,  at  least  with  present  exhaust 
methods.  It  would,  furthermore,  make  it  necessary  to  take  the  roentgen  rays  out 


Fig.  20.     Diagram  showing  Arrangement  of  Apparatus  for 
Determining  Effectiveness  of  Different  Methods 


103 


through  the  cathode  structure.  The  placing  of  the  cathode  too  close  to  the  anode  would 
also  make  it  difficult  to  maintain  a  constant  filament  temperature,  that  is,  if  the  target 
were  allowed  to  become  incandescent. 

The  use  of  a  metal  of  low  atomic  weight  for  the  surface  of  the  entire  target  (exclusive 
of  that  needed  for  the  focal  spot)  and  stem,  is  practicable  only  in  case  the  target  is  not 
to  be  allowed  to  become  very  hot,  for  the  reason  that  there  is  no  metal  of  low  atomic 
weight  which  is  sufficiently  refractory.  (Among  the  non-metallic  elements,  carbon 
might  possibly  be  used,  but  it  would  render  the  exhaust  more  difficult,  and  at  high  tem- 
perature it  furthermore  alloys  readily  with  tungsten.) 


•  ••  t ft 

•  •  • ttt 

•••ttt 


Relative    Test    of  Effectiveness   of  the 
Various  Methods 

Of  course,  the  pinhole-camera 
roentgenograms  themselves  give  some 
idea  of  the  effectiveness  of  the  various 
methods.  Nevertheless,  it  is  difficult 
to  say  from  an  inspection  of  them 
which  method  would  give  a  tube  that 
would  make  of  a  given  object  the  best 
roentgenogram.  For  this  reason  a 
test  method,  which  has  been  described 
before*  and  which  is  illustrated  in  Fig. 

Fig.  21.     Left,  Roentgenogram  through  Holes  in  Lead  Plate  20'  Was  TCSOrted  to.       The  test  object 

with  Hooded  Target  (Fig.  8) ;  Right,  Standard  Tube.  is  a  lead  plate  with   nine  Small  TOUnd 

holes  in  it,  and  is  placed  at  a  distance 

of  2^4  in.  from  the  photographic  plate.  The  tube  is  placed  with  its  focal  spot  20 
in.  from  the  X-ray  plate.  No  external  diaphragm  or  cone  is  used.  All  exposures  are 
made  with  the  same  voltage,  current 
and  time,  and  the  same  develop- 
ment is  used  throughout.  If  all  of 
the  roentgen  rays  came  from  the  focal 
spot,  the  radiogram  would,  in  each 
case,  be  a  series  of  sharply  defined 
circular  areas.  Any  rays  coming  from 
the  body  and  stem  of  the  target  tend 
to  produce  a  halo  on  one  side  of  each 
of  these  circles.  (^D 

The  roentgenogram   to  the  right 
in  Fig.  21  was  made  with  the  stand-       ^_ 
ard  tube  and  that  to  the  left  with  the 
tube  having  a  hooded  target. 

Fig.  22  shows  a  similar  comparison 

between  the  standard  tube  and  the  one  with  the  target  set  into  the  side  of  the  bulb, 
as  in  Fig.  16. 

Fig.  23  is  a  test  of  the  standard  tube  versus  the  one  with  the  internal  diaphragm, 
as  in  Fig.  18. 

It  will  be  seen  that,  according  to  this  test,  the  tube  with  the  internal  diaphragm  is 
the  best  of  those  tried.    The  tube  with  the  copper-backed  target  was  not  included.    The 

*  W.  D.  Coolidge,  Am.  Journal  of  Roentgenology,  Vol.  II,  p.  885  (1915). 

104 


•••tit 
•••ttt 

••••09 


one  with  the  cathode  placed  very  close  to  the  anode,  as  in  Fig.  13,  was  not  included  for 
the  reason  that,  while  from  the  body  and  stem  of  the  target  it  gave  the  least  radiation 
of  any  of  the  tubes,  the  design  of  this  particular  tube  was  not  adapted  to  the  test. 


ttt 

tit 
ttt 


Comparative    Test   of  the    Tube    with 

Internal    Diaphragm    Against    the 

Standard  Tube  in  Medical   Roent- 

genography 

The  test  with  the  lead  plate  with 
the  holes  was  recognized  in  the  begin- 
ning  as  an  artificial  one,  the  results  of 
which  might  be  difficult  to  interpret 
in  terms  of  medical  work.  It  seemed 
to  be  a  safe  guide,  however,  as  to 
the  relative  effectiveness  of  the  dif- 
ferent methods.  — 

Having  determined  in  this  manner  Fig.  23.     Left,  Tube  with  Internal  Diaphragm  (Fig.  18). 

that,    of    the    various    experimental  Right,  standard  Tube 

tubes  made,  the  one  with  the  internal 

diaphragm  should  be  looked  to  for  the  best  definition  in  roent genography,  the  next 

problem  was  to  show  by  actual  tests  whether  roentgenographs  of  medical  subjects 

made  with  the  internally  diaphragmed  tube  were  appreciably  better  than  those  made 

with  the  standard  tube. 

A  pair  of  tubes  was  chosen,  one  with  and  the  other  without  the  diaphragm,  with  the 
same  size  of  focal  spot  (as  determined  by  the  pinhole-camera  method) . 

The  difference  found  in  the  pairs  of  plates  made  with  these  tubes  of  various  parts  of 
the  body,  was  so  small  that  it  proved  very  difficult  to  show  beyond  reasonable  doubt 
that  there  was  really  any  perceptible  difference.  Failure  to  get  absolute  immobilization 
of  the  part  in  question  with  either  roentgenogram  was  enough  to  bring  a  decision  in 
favor  of  the  other  plate.  It  was  finally  found  that  whatever  difference  there  is  could 
better  be  shown  by  roentgenograms  of  a  dried  skull.  This  test  appears  to  be  more 
searching  than  that  with  a  live  subject,  for  the  reason  that  the  smaller  amount  of 
secondary  radiation  from  the  dried  skull  gives  better  definition  and  hence  puts  the  ob- 
server in  a  favorable  position  to  notice  differences  ascribable  to  the  cause  in  question. 
To  further  this  same  end,  a  very  small  size  of  focal  spot  was  chosen  for  the  test.  The 
most  important  thing  of  all  was  to  use  exactly  the  same  voltage  on  both  tubes.  This 
was  accomplished  by  the  use  of  auto-transformer  control  of  the  high-voltage  transformer 
and  by  the  use  of  a  stabilizer  in  the  filament  circuit. 

The  difference  between  such  a  pair  of  plates  is  so  small  that  it  at  first  escaped  de- 
tection. It  would  be  lost  in  reproduction.  Such  a  pair  of  plates  has  been  submitted  to 
quite  a  number  of  prominent  roentgenologists,  and  some  of  these  have  chosen  the  right 
plate  and  some  the  wrong  one  as  the  work  of  the  tube  with  the  internal  diaphragm. 

Explanation  of  the  Smallness  of  the  Gain  Made  in  the  Quality  of  the  Roentgenogram  by  Re- 
ducing the  Effect  of  the  Radiation  from  the  Target  Outside  of  the  Focal  Spot. 

The  explanation  lies  in  the  sensitiveness  curve  of  the  photographic  plate  and  in  the 
narrow  range  of  plate  densities  involved  in  roentgenograms  of  the  human  body.  This 
is  easily  proved  by  going  back  and  making  further  roentgenograms  of  the  lead  plate 

105 


with  the  holes. 


If,   instead  of  greatly  over-exposing  the   area  of  the  plate  under 

the  holes,  so  as  to  bring  out  the  halos 
"1  strongly,  the  exposure  is  so  timed  as 
to  bring  the  density  of  this  part  of 
the  developed  plate  only  to  the  maxi- 
mum  density  found  in  actual  roent- 
genograms  of  the  human  body,  it 
becomes  difficult  to  see  the  halos. 
This  is  illustrated  by  Figs.  24  and  25. 
These  were  made  with  the  pair  of 
tubes  in  question,  one  with  and  one 
without  an  internal  diaphragm,  and 
with  the  same  technique  employed  in 
Figs.  21,  22,  and  23,  except  that 
the  exposure,  instead  of  being  37.5 
milliampere  seconds,  was  10  in  Fig. 
24  and  1  in  Fig.  25. 


Fig.  24.     Left,  Tube  with  Internal  Diaphragm 

Right,  Standard  Tube.     Exposure,  1C 

Milliampere  Seconds 


Conclusions  Concerning  the  Importance  of  the  Role  Played  by  the  Rays  from  the  Target. 
Outside  of  the  Focal  Spot,  in  Medical  Diagnosis 

With  the  present  roentgeno- 
graphic  technique,  the  part  played 
by  these  rays  seems  to  the  writers 
to  be  too  small  to  warrant  the  use 
of  any  one  of  the  methods  described 
for  minimizing  it,  that  is,  with  the 
present  standard  type  of  tube. 

In  fluoroscopy,  comparative  tests 
have  not  yet  been  made  between 
the  standard  and  the  internally  dia- 
phragmed  tube;  but  it  seems  most 
probable  that  the  situation  is  not 
essentially  different  here  from  what  Fig.  25.  Left,  Tube  with  internal  Diaphragm ; 

Right,  Standard  Tube.     Exposure,  1 
It  IS  in  roentgenography.  Milliampere  Second 


106 


A  PORTABLE  ROENTGEN-RAY  GENERATING  OUTFIT* 
BY  \V.  D.  COOLIDGE  AND  C.  N.  MOORE 

INTRODUCTION 

When  it  appeared  that  this  country  might  become  involved  in  the  European  war 
various  Red  Cross  Units  were  formed,  and  some  of  these  instituted  inquiries  concerning 
Roentgen-ray  apparatus  suitable  for  war  needs.  It  was  through  such  inquiries  that 
the  authors  became  interested  in  the  problem  of  a  portable  Roentgen-ray  generating 
outfit. 

The  problem  as  it  first  presented  itself  appeared  rather  indefinite,  as  but  little  was 
generally  known  concerning  the  degree  of  portability  required,  the  current  and  voltage 
needed  to  operate  the  tube,  the  amount  of  service  which  would  be  required  of  the  ap- 
paratus, the  question  as  to  whether  power  could  usually  be  had  from  existing  electric 
circuits,  etc.f  Existing  outfits  apparently  left  much  to  be  desired.  The  best  way  of 
attacking  the  problem  seemed  to  be  to  investigate  the  various  possible  systems,  so  as 
to  determine  which  was  best  adapted  for  development  into  a  generating  outfit  of  con- 
siderable portability  and  sufficient  output. 


The  various  systems  which  the  authors  seriously  considered  for  the  purpose  were : 

(1)  A    gasolene-electric  set  furnishing  low  voltage  direct   current   to  a  rotary 

converter,  with  a  step-up  transformer  and  a  mechanical  rectifier  attached 
to  the  shaft  of  the  rotary. 

(2)  A  gasolene-electric  set  furnishing  alternating  current  to  a  step-up  transformer 

and  with  a  mechanical  rectifier  driven  from  a  small  synchronous  motor. 

(3)  The  same  as  (2) ,  except  for  the  substitution  of  a  kenotron  for  the  synchronous 

motor  and  mechanical  rectifier. 

(4)  A  gasolene-electric  set  furnishing  alternating  current  to  a  step-up  transformer, 

with  a  self-rectifying  Roentgen-ray  tube  operating  directly  from  the  latter. 

(5)  A  storage  battery  operating  an  induction  coil  through  a  mechanical  interrupter 

or  a  mercury-turbine  interrupter  with  suitable  gas  dielectric. 

(6)  A  storage  battery  operating  a  motor-generator  and  so  producing  alternating 

current  to  be  fed  to  a  step-up  transformer  and  thence  to  a  self -rectify  ing 
Roentgen-ray  tube. 

An  investigation  of  these  separate  methods  led  to  the  elimination  of  all  but  (4)  for 
the  following  reasons : 

With  (1),  to  avoid  the  use  of  long  high-tension  leads,  it  appeared  necessary  to  locate 
the  gasolene-electric  set  close  to  the  Roentgen-ray  table,  and  hence  to  make  the  operat- 
ing room  noisy.  Furthermore,  the  efficiency  of  a  small  rotary  is  very  low  for  the  size  in 
question,  perhaps  25  per  cent,  and  this  would  necessitate  the  use  of  a  relatively  large 
and  heavy  gasolene-electric  set. 

*  From  General  Electric  Review,  XXI,  January,  1918,  pp.  60-67.  Since  this  article  was  published  there  have  been  some 
changes  made  in  the  portable  table  and  transformer  box,  so  that  the  present  form  of  these  devices  does  not  conform 
with  those  shown  in  Fig.  4  and  Fig.  5  of  this  article. 

t  Among  others,  the  following  excellent  papers  bearing  on  the  subject  were  read  with  great  interest : 
"The  X-ray  Equipment  and  Work  in  the  Army  at  the  Present  Time,"  by  Capt.  William  A.  Duncan,  American  Journal 
of  Roentgenology,  268-275  (1914). 

"The    Electrotechnical    Problem    of    the    Production    of    Roentgen  Rays  in  the    Present    War."  by    Dr.   Umberto 
Magini,  L'Electrotechnica,  Vol.  ///,  No.  7,  March  5,  pp.  126-133  (1916). 
Copyright,  1918,  by  General  Electric  Review. 

107 


There  were  numerous  objections  to  (2).  It  practically  necessitated  placing  the 
rectifier  near  the  operating  table  where  the  noise  was  undesirable;  the  use  of  the  syn- 
chronously driven  mechanical  rectifier  involved  the  loss  of  a  considerable  amount  of 
energy  (the  small  one  which  we  tried  consumed  350  watts) ;  the  behavior  of  this  rectifier 
on  the  current  supplied  by  a  dynamo  driven  by  a  small  single-cylinder,  4-cycle  engine 
was  not  good;  and  finally,  the  rectifier  looked  like  a  serious  complication  provided  it 
could  be  dispensed  with. 

While  (3)  was  good,  it  was  clearly  not  as  simple  as  (4). 

It  developed  that  the  electrical  efficiency  of  both  (5)  and  (6)  was  very  low,  probably 
less  than  25  per  cent.  This  meant  that,  for  the  production  of  any  reasonable  Roentgen- 
ray  intensity  for  any  considerable  length  of  time,  either  of  these  systems  would  lead  to 
a  relatively  heavy  outfit. 

(a) 


(b) 


D 


Fig.  1.     Separate  Units  Composing  Portable  Roentgen-ray  Outfit 


THE  SYSTEM  CHOSEN 

System  (4),  consisting  of  a  gasolene-electric  set  furnishing  alternating  current  to  a 
step-up  transformer,  and  with  a  special  self-rectifying  Roentgen-ray  tube  operating  di- 
rectly from  the  latter,  was  finally  chosen.    The  advantages  of  this  system  when  com- 
pared with  the  others  may  be  briefly  summarized  as  follows : 
It  is  simpler. 

It  is  more  efficient  electrically. 

It  is  lighter  in  weight  for  a  given  output,  and  hence  more  portable. 
It  does  not  involve  the  care  of  a  storage  battery. 

It  involves  the  use  of  no  moving  parts  other  than  the  gasolene-electric  set. 
The  outfit  is  self-contained,  requiring  merely  gasolene  to  run  it,  and  hence  can  be 
operated  anywhere  at  any  time  regardless  of  the  presence  or  absence  of  electric 
supply  circuits. 

The  one  member  having  moving  parts,  namely  the  gasolene-electric  set,  can  be 
located  at  any  desired  distance  from  the  Roentgen-ray  room. 

108 


THE  OUTFIT  AS  DEVELOPED 
Separate  Elements  Comprising  the  Outfit 

Having  settled  upon  the  system,  it  became  necessary  to  get  the  component  parts. 
The  exigency  of  the  case  made  it  desirable  to  use,  in  so  far  as  possible,  apparatus  which 
was  already  in  production  and  hence  readily  available.  For  this  reason,  apparatus  was 
borrowed  from  many  different  manufacturers  and  tested  for  its  fitness  for  the  particular 
purpose  in  question.  The  elements  which  were  finally  chosen  are  shown  in  Fig.  1.  A 
description  of  each  of  these  elements,  together  with  the  reasons  for  choosing  it,  is  given 
in  the  following: 

(A}  Gasolene-electric  Set 

Various  single-  and  multiple-cylinder  engines  of  the  2-cycle  and  4-cycle  type  were 
tried  out,  and  the  conclusion  was  reached  that  the  one  which  was  best  adapted  to  the 
purpose  was  the  Delco-light  engine  made  by  the  Domestic  Engineering  Company  of 
Dayton,  Ohio.  This  engine  was  already  direct-connected  to  a  dynamo. 

The  Delco-light  set  was  originally  built  for  direct  current  at  40  volts,  but  by  a 
change  in  armature-  and  field-windings  and  by  the  addition  of  a  pair  of  slip  rings  and 
brushes  it  was  adapted  to  furnish  alternating  current  at  the  desired  voltage.  The  en- 
gine was  also  provided  with  a  special  throttle-governor  for  regulating  the  generator 
voltage.  This  governor  consists  of  a  solenoid  (a)  mounted  above  the  carburetor,  the 
movable  core  of  the  solenoid  being  connected  to  the  butterfly  valve  of  the  throttle.  The 
solenoid  is  operated  by  direct  current  taken  from  the  commutator  of  the  generator.  A 


Fig.  2.     Special  Self-rectifying  Roentgen-ray  Tube 

variable  resistance  (b)  is  placed  in  series  with  the  solenoid,  and  by  means  of  this  re- 
sistance any  throttle  opening  and  hence  any  desired  engine  speed  and  alternating  cur- 
rent voltage  may  be  obtained.  The  alternating-current  voltage  is  indicated  by  the 
voltmeter  (c). 

The  reasons  for  choosing  the  Delco-light  outfit  in  preference  to  others  were: 

It  has  a  single  cylinder  engine  with  corresponding  mechanical  simplicity. 

The  engine  is  of  the  4-cycle  type  and  hence  easy  to  start  and  flexible  in  operation. 

The  engine  is  air-cooled,  and  this  does  away  with  the  possibility  of  having  water 
freeze  in  the  jackets  in  cold  weather. 

The  armature  of  the  dynamo  is  mounted  directly  on  the  crankshaft  of  the  engine 
and  the  crankshaft  has  but  two  main  bearings.  This  effectively  does  away  with 
coupling  troubles  and  with  lack  of  alignment. 

It  is  self-lubricating. 

It  is  self-excited. 

It  is  of  suitable  capacity. 

It  has  been  developed  for  long  continued  service  with  a  minimum  of  attention. 

At  great  expense,  it  has  been  thoroughly  standardized,  and  this  means  inter- 
changeability  of  parts,  and  that  all  sets  will  have  essentially  the  same  characteristics . 

The  workmanship  is  excellent. 

It  is  available  in  any  desired  quantity  on  very  short  notice. 

109 


(B)  Roentgen-ray  Transformer 

A  small  transformer,  made  by  the  Victor  Electric  Company  of  Chicago,  for  their 
dental  radiographic  outfit,  was  chosen  for  the  following  reasons : 

It  is  an  oil-insulated  closed-magnetic  circuit  transformer  of  the  proper  capacity. 

It  is  oil-tight. 

Its  design  is  such  that,  with  the  load  in  question,  the  "inverse"  voltage  is  not  pro- 
hibitively high. 

It  was  available  on  short  notice,  as  only  two  minor  changes  were  required.  These 
changes  consisted  in  adapting  the  primary  winding  to  the  voltage  of  the  gener- 
ator and  in  bringing  out  leads  for  the  milliammeter  (d)  from  the  grounded  mid- 
dle point  of  the  secondary  (this  makes  the  milliammeter  a  low-tension  instrument) . 

(C)  Filament-current  Transformer 

Here  again  it  seemed  imperative  that  an  oil  insulated  transformer  should  be  used, 
and  that  this  should  be  electrically  efficient,  of  light  weight,  and  oil  tight. 

The  Victor  filament-current  transformer  appeared  to  fulfill  these  conditions  better 
than  any  other  which  was  available  at  the  time. 

(D)  Ftlament-current  Control 

That  manufactured  by  the  Wappler  Electric  Company  was  chosen  for  this  outfit 
because  of  its  small  size,  fineness  of  regulation,  and  the  fact  that  the  setting,  once  made, 
was  not  easily  disturbed. 

(E)  Booster 

The  line  voltage  drops  considerably  when  the  full  load  is  thrown  on  the  generator. 
To  prevent  the  lowering  of  the  filament-current  which  would  result  from  this,  a  small 
transformer  was  designed,  the  primary  of  which  is  inserted  in  the  primary  circuit  of  the 
X-ray  transformer  and  the  secondary  in  the  primary  circuit  of  the  filament-trans- 
former, as  shown  in  the  wiring  diagram  (Fig.  3). 

(F)  Operating  Switch 

A  single-pole  single-throw  switch  of  substantial  construction  was  chosen. 

(G)  Special  Self-rectifying  Tube 

There  was  no  tube  suitable  for  diagnostic  work  and  capable  of  rectifying  its  own  cur- 
rent, for  frequent  long  exposures,  with  as  much  energy  as  that  available  from  the  Delco- 
light  set.  It,  therefore,. became  necessary  to  undertake  its  development,  and  the  result 
is  seen  in  Fig.  2.  This  tube  is  fully  discussed  in  a  separate  paper  published  in  the  cur- 
rent number  of  the  Review  by  one  of  the  authors.  Briefly,  it  is  a  hot-cathode  tube  with 
a  9.5  cm.  (3%  in.)  bulb.  The  cathode  has  been  especially  designed  to  give  a  focal  spot 
3.2  mm.  (%  in.)  in  diameter  and  with  a  very  uniform  distribution  of  energy.  The  target 
consists  of  a  small  wrought-tungsten  button  set  in  a  solid  block  of  copper,  and  this  block 
is  electrically  welded  to  a  solid  copper  stem  1.6  cm.  (%  in.)  in  diameter  which  extends 
'  out  through  the  anode  arm  to  an  external  radiator.  A  platinum  sleeve  is  silver-soldered 
,at  one  end  to  the  copper  stem  and  attached  at  the  other  end  to  the  glass  of  the  anode 
arm.  The  target,  complete  with  stem  and  radiator,  weight  860  gms.  and  has  a  heat 
capacity  of  81  calories  per  "degree  centigrade.  The  present  standard  solid  tungsten  tar- 
get, complete  with  molybdenum  stem  and  iron  supporting  tube,  has  a  heat  capacity  of 
less  than  10  calories.  Because  of  its  greater  heat  capacity,  it  takes  much  longer  to  heat 
the  radiator  type  of  target  to  redness  than  it  does  the  solid  tungsten  target.  Unlike  the 
latter,  the  target  in  the  new  tube,  even  at  relatively  low  temperatures,  cools  rapidly  in 
the  interval  between  radiographic  exposures,  and,  therefore,  permits  of  starting  each 
exposure  with  an  essentially  cold  target.  As  a  result,  the  focal  spot,  even  though  small, 
is  kept  from  reaching  a  temperature  high  enough  to  allow  "inverse"  current  to  pass 

110 


/tOff/TSlN  KAY  TUBE 


HOCNT6CN  KAY 


RO£NTa£N  KAY  SWITCH- 


'•:  -F/LAHENT  TKAHSFORMCK 


— FILAMENT  CONTKOL 


through  the  tube.  Furthermore  (and  this  serves  as  an  additional  safeguard),  this  type 
of  tube  does  not  allow  an  appreciable  amount  of  inverse  current  to  pass  even  though 
it  is  so  badly  overloaded  that  the  focal  spot  becomes  heated  to  the  melting  point.  A 
probable  explanation  of  this  striking  fact 
is  given  in  the  companion  article  on  the 
tube. 

COMPLETE  WIRING  DIAGRAM 
This  is  shown  in  Fig.  3  and  needs  no 
further  explanation. 

COMPLETE  OUTFIT 
The  complete  outfit  is  shown  in  Fig. 
4,  in  which  the  gasolene-electric  set  is 
seen  at  the  left.  The  Roentgen-ray  and 
filament-current  transformers,  the  fila- 
ment current  control,  and  the  booster  are 
in  the  lower  part  of  the  box  at  the  end 
of  the  table  (see  Fig.  5,  which  shows  the 
inside  of  this  box) .  On  a  shelf  in  the  top 
of  this  box  are  a  voltmeter  for  showing 
line  voltage,  the  adjustable  rheostat  for 
controlling  line  voltage,  a  milliammeter 
for  indicating  the  tube  current,  and  the 
operating  switch.  (There  is  a  cover  for 
this  box  which  is  screwed  on  for  ship- 
ment.) The  X-ray  tube  is  permanently 
located  in  the  movable  tube-box  under 

the  table.  Fig.  3.     Complete  Wiring  Diagram    "J 


Fig.  4.     Complete  Portable  Roentgen-ray  Outfit 

TECHNIQUE 

The  outfit  as  described  lends  itself  readily  to  a  very  simple  technique.    Experience 
extending  over  several  years  has  convinced  the  writers  that  it  is  very  convenient,  for 

*  The  table,  tube  box,  and  shutter  were  all  developed  by  the  co-operative  work  of  others,  including  Major  Shearer, 
Major  Geo.  Johnston,  and  the  Kelley-Koett  Company. 

Ill 


experimental  work,  to  have  extreme  flexibility  in  a  Roentgen-ray  generating  outfit,  so 
that  the  penetrating  power  of  the  rays  can  be  varied  at  will  between  wide  limits.  It  has 
also  convinced  them,  however,  that  Roentgenologists  would  get  better  average  results 
in  diagnostic  work  if  their  outfits  were  made  much  less  flexible,  so  that  the  Roentgen-ray 
tube  could  be  used  more,  for  example,  as  the  ordinary  incandescent  lamp  is.  When  such 
a  lamp  is  lighted  on  the  ordinary  constant-potential  circuit,  it  gives  always  the  same 
kind  and  the  same  amount  of  light.  It  is  entirely  practicable  to  operate  a  Roentgen- 
ray  tube  in  this  same  manner.  It  is  merely  necessary  to  see  that  the  filament  current  is 
always  the  same  and  that  the  high-tension  voltage  impressed  upon  the  tube  terminals 
is  always  the  same.  Under  these  circumstances  the  tube  always  gives  out  Roentgen- 
rays  of  the  same  penetrating  power*  and  of  the  same  intensity.  More  artistic  radio- 
graphs can  be  made  by  adapting  the  penetrating  power  of  the  rays  to  the  thickness  of 
the  part  to  be  radiographed,  but  experience  shows  that  with  a  suitable  compromise 
voltage  (we  have,  adopted  that  corresponding  to  a  5-in.  spark  between  pointed  elec- 
trodes which  is  equal  to  57,500  volts  effective,  as  actually  measured  by  a  sphere  gap) 


ffoentgen  Ray  Tube 


Roentgen  /?ay_V 
Transformer 


Roentgen  ffa/j 
Switch 


^filament 
Transformer 


~f~i/ament  Control 


Fig.  5.     Transformer  and  Instrument  Box  Fig.   6.     Wiring    Diagram    for    Outfit    Operated    Directly    from 

Existing  Alternating    Current    Mains   (when 
Gasolene  Electric  Set  is  not  needed) 

4fef 

excellent  radiographs  can  be  made  of  all  parts  of  the  human  body,  and  with  a  great 
simplification  in  apparatus  and  technique.  The  time  factor  becomes  the  only  variable 
and  this  is  adapted  to  the  thickness  of  the  part  to  be  radiographed. 

With  57,500  volts  (effective)  and  10  milliamperes  and  a  distance  of  45.7  cm.  (IS  in.) 
from  focal  spot  to  plate,  the  writers  have  found  that  for  Seed  plates  and  the  average 
adult  subject,  the  following  table  of  exposures  gives  good  results: 


Hand. 
Elbow . 
Knee.  . 


Exposures  18  in.  distance 

1  Shoulder 

2  Hip 

4  Frontal  sinus. 


8 
20 
50 


Too  much  weight  should  not  be  attached  to  this  table,  as  it  is  based  on  a  relatively 
small  amount  of  work.    It  is  merely  given  to  show  in  a  general  way  how  the  time  of  ex- 


*  These  rays  are  not  homogeneous,  but  the  mixture  is  always  the  same. 

112 


V 


posure  varies  for  different  parts  of  the  body  when  the  intensity  and  penetrating  power 
of  the  rays  are  held  constant. 

The  compromise  voltage  used  for  radiography  appears  well  suited  to  fluoroscopy, 
and  at  this  voltage  a  current  of  5  ma.  appears  to  give  sufficient  illumination. 

The  reason  for  adopting  for  radiography  the  amount  of  energy  corresponding  to  10 
ma.  at  a  5-in.  spark  was  that  it  was  the  maximum  amount  available  in  the  tube  when 
operating  from  the  Delco-light  generator. 

This  energy  could,  of 'course,  have  been  taken  at  any  desired  potential,  as  this  was 
merely  a  matter  of  transformer  design.  The  reasons  for  choosing  the  potential  cor- 
responding to  a  5-in.  spark  were:  Given  a  certain  amount  of  energy  to  work  with,  the 
Roentgen-ray  intensity,  as  measured  by  the  illumination  of  the  fluorescent  screen  or 
the  action  on  the  photographic  plate,  goes  up  rapidly  with  the  voltage.  This  is  obvious 
from  the  well  known  fact  that  Roentgen-ray  intensity,  measured  as  above,  increases 
with  the  first  power  of  the  milliamperage  and  with  the  square  of  the  voltage  while  the 
energy  delivered  to  the  tube  is  proportional  to  the  product  of  the  first  power  of  the  cur- 
tent  and  the  voltage.  This,  then,  was  an  argument  in  favor  of  high  voltage.  On  the 
other  hand,  contrast,  in  both  radiography  and  fluoroscopy,  decreases  with  increasing 
voltage.  This  is  an  argument  against  the  use  of  too  high  voltage.  The  voltage  cor- 
responding to  a  5-in.  spark  between  points  appeared  to  be  a  good  compromise. 

The  authors  have  adopted  the  following  general  method  of  using  the  outfit. 

After  starting  the  engine,  the  resistance  (b)  is  all  cut  out.  This  causes  the  engine 
to  idle  at  its  lowest  speed. 

For  radiographic  work,  the  resistance  of  (b)  is  raised  until  the  line  voltage  indicated 
by  (c)  is  about  160.  The  filament  current  is  then  adjusted  once  for  all*  by  means  of  the 
control  (D)  until,  upon  closing  the  X-ray  switch,  the  milliamperemeter  registers  10. 
As  the  X-ray  switch  is  closed  the  line  voltage  is  seen  to  drop.  The  resistance  of  (b) 
should  be  changed  until  the  line  voltage  with  the  10  ma.  load,  is  122.  The  line  voltage 
is  now  observed  when  the  load  is  taken  off,  and  is  in  future  work  brought  to  this  point 
before  closing  the  Roentgen-ray  switch.  The  important  point  is  that  the  voltmeter 
reading  when  the  10  milliampere  load  is  on  shall  always  be  122.  If  this  condition  is  ful- 
filled, all  radiographic  work  will  be  done  with  a  tube  voltage  corresponding  to  a  5-in. 
spark  between  points. 

For  fluoroscopic  work,  it  is  not  necessary  to  touch  the  filament  current  control. 
This  should  be  left  just  as  it  was  for  radiographic  work.  After  closing  the  Roentgen-ray 
switch,  the  rheostat  (b)  is  so  adjusted  that  the  tube  carries  5  milliamperes.  The  tube 
voltage  will  then  be  the  same  as  it  is  with  the  10-ma.  setting  for  radiographic  work. 

ENGINEERING  DATA 
(/)  Electrical  Efficiency 

Wattmeter  measurements  taken  at  various  points  in  the  circuit  showed  that,  with 
the  radiographic  load  of  10  ma.  at  57,500  volts  (effective),  the  alternating-current  gen- 
erator was  delivering  820  watts,  and  that  this  energy  was  consumed  in  various  parts  of 

the  outfit  as  follows: 

Watts 

Main  line  loss 10 

Booster  loss 12 

Filament-current  control  loss 14 

Filament-current  transformer  loss 5 

Energy  consumed  in  filament 43 

Energy  delivered  to  Roentgen-ray  transformer 694 

Total..  778 


*   It  should  subsequently  need  to  be  changed  only  when  tubes  are  changed. 

113 


The  difference  of  5  per  cent  between  this  total  and  the  820  watts  measured  directly 
at  the  brushes  of  the  generator  is  doubtless  to  be  explained  in  part  by  experimental 
error  and  in  part  by  distortion  of  wave-form. 

From  the  above,  it  is  seen  that  85  per  cent  (694  watts)  of  the  energy  from  the  gen- 
erator is  delivered  to  the  Roentgen-ray  transformer. 

The  energy  in  the  high-tension  discharge  passing  through  the  tube  was  not  measured 
directly.  (If  time  had  permitted,  it  would  have  been  determined  by  thermal  measure- 
ments of  the  heat  generated  in  the  target.)  It  should  be  approximately  equal  to  575 
watts  (the  product  of  tube  current  and  voltage,  or  0.010  by  57,500.)  According  to  this, 
the  Roentgen-ray  tube  gets,  in  the  form  of  high-tension  discharge,  83  per  cent  of  the  energy 
delivered  to  the  Roentgen-ray  transformer  and  70  per  cent  of  the  total  energy  delivered  by  the 
alternating-current  generator. 
(2}  Gasolene  Economy 

This  was  determined  for  various  Roentgen-ray  loads.  In  the  following  table,  the 
first  column  gives  the  number  of  the  experiment,  the  second  the  load,  the  third  the  en- 
gine speed  in  revolutions  per  minute,  the  fourth  the  dynamo  voltage,  and  the  last  the 
economy  expressed  in  hours  of  operation  per  gallon  of  gasolene  consumed. 


Expt. 
Xo. 

Roentgen-ray  Load 

R.p.m. 

Dynamo 
Voltage 

Hours  per 

Gallon 

1 

0 

900 

90 

5yz 

2 

0 

1440 

160 

4 

3 

4 

Radiographic 
Fluoroscopic 

1382 
1320 

122 
114 

3j| 

In  Experiment  1,  the  engine  was  idling  at  its  lowest  speed  and  the  Roentgen-ray 
switch  was  open.  In  Experiment  2,  the  engine  speed  was  that  required  tor  radiographic 
work,  but  the  Roentgen-ray  switch  was  open.  In  Experiment  3,  the  radiographic  load 
is  on  for  15  seconds  and  then  off  for  the  remaining  45  seconds  of  each  minute.  (This 
particular  schedule  was  chosen  as  representing  the  severest  radiographic  service  which 
was  likely  to  be  required  of  the  outfit.)  The  fourth  experiment  was  with  a  fluoroscopic 
load  kept  on  continuously. 
(3}  Weights 

The  weight  in  pounds  of  the  different  parts  of  the  outfit  is  given  in  the  following 
table : 

Engine  and  generator,  with  wooden  base 377  Ibs. 

Tube-box,  and  shutter 110  Ibs. 

Table 164  Ibs. 

Transformer  and  instrument  box  with  contents 244  Ibs. 

Total 895  Ibs. 

ADVANTAGES  OF  THE  OUTFIT 

The  advantages  of  the  complete  outfit,  as  described  above,  may  be  summarized  as 
follows : 

(1)  Simplicity  of  apparatus. 

(2)  High  efficiency  with  consequent  light  weight  and  portability. 

(3)  No  moving  parts  other  than  the  gasolene-electric  set. 

(4)  Control  of  line  voltage,  making  it  possible  to  duplicate  electrical  conditions 

and  X-ray  results  very  accurately. 

(5)  The  tube  has  been  designed  to  carry  all  of  the  energy  that  the  generator  can 

deliver.    For  this  reason  no  harm  can  be  done  to  the  tube  by  raising  the 


114 


filament  current  too  high.  Under  such  conditions  the  milliamperage  will  go 
up,  but  the  voltage  will  decrease  so  that  the  energy  delivered  to  the  tube 
will  not  go  up  appreciably. 

(6)  The  gasolene-electric  set  can  be  placed  at  any  desired  distance  from  the  high 

tension  part  of  the  outfit,  so  as  to  avoid  noise  in  the  operating  room. 

(7)  No  storage  battery  to  get  out  of  order. 

(8)  Engine  can  be  made  to  idle  at  very  low  speed,  conducive  to  long  life.    This 

advantage  comes  from  the  use  of  the  voltage  governor  instead  of  a  speed 
governor. 

(9)  An  accidental  short  circuit  of  the  generator  does  no  harm;  it  simply  lowers 

the  line  voltage  to  such  an  extent  that  the  field  excitation  goes  down,  the 
ignition  fails,  and  the  engine  stops. 

IMPROVEMENTS  WHICH  COULD  BE  MADE  BY  SPECIAL  DESIGN 

The  outfit  described  was  deve'oped  primarily  for  army  use,  and  the  exigency  of 
the  case  made  it  necessary,  in  so  far  as  possible,  to  make  use  of  parts  which  were 
already  in  production  and  hence  readily  available.  It  could  be  further  simplified 
and  improved  by  special  design. 

The  greatest  improvement  would  come  from  a  reduction  in  the  "inverse"  voltage. 
By  proper  electrical  design  of  the  dynamo  and  the  high-voltage  transformer,  this  "in- 
verse" voltage  could  readily  be  reduced  to  a  point  where  it  would  be  but  slightly  in  ex- 
cess of  the  "useful"  voltage. 

This  would  make  it  practicable  and  easy  to  work  with  a  high-tension  system 
grounded  on  one  side.  The  cathode  of  the  tube  would  then  be  connected  to  the  lead 
covering  of  the  tube  box,  and  through  this  and  the  supporting  mechanisms  to  the 
grounded  metal  frame  of  the  table.  There  would  then  be  only  one  high-tension  ter- 
minal to  the  transformer  and  only  one  high-tension  wire  going  to  the  tube  (to  the 
anode) .  The  grounding  of  the  cathode  end  of  the  tube  would  make  it  possible  to  dis- 
pense with  the  relatively  bulky  filament-current  transformer  insulated  for  high  poten- 
tial and  to  replace  it  with  a  very  small  ordinary  low- voltage  transformer.  It  would  also, 
for  the  following  reasons,  increase  the  allowable  tube  travel :  first,  the  grounded  end  of 
the  tube  could  safely  be  moved  to  the  extreme  end  of  the  table ;  and,  second,  owing  to 
the  reduction  in  inverse  voltage,  the  length  of  tube  and  tube-box  could  be  materially 
reduced. 

The  set  described  is  intended  to  be  used  as  a  unit ;  but  it  would  be  a  simple  matter 
to  so  modify  it  that  it  could  be  operated  from  existing  electric  circuits  without  running 
the  gasolene-electric  set.  The  wiring  diagram  (Fig.  6)  shows  the  simplicity  of  such  a 
system.  Where  different  line  voltages  are  to  be  encountered,  the  Roentgen-ray  and 
filament-current  transformers  would  have  special  primary  windings  with  several  taps, 
or  with  several  sections  which  could  be  connected  in  series  or  in  multiple.  Another 
method  of  accomplishing  the  same  result  would  consist  in  using  an  auto-transformer 
provided  with  suitable  taps  to  take  in  current  of  any  commercial  voltage  and  deliver  it 
at  the  122  volts  given  by  the  Delco-light  set.  There  is  already  sufficient  iron  in  the 
transformers  to  take  care  of  a  wide  range  of  frequencies.  For  operation  from  direct- 
current  mains,  a  rotary  converter  would  be  needed.  When  operating  from  existing 
electric  mains,  with  either  alternating  or  direct  current,  the  booster  would  not  be 
needed. 

In  closing,  the  authors  wish  to  express  their  thanks  to  the  many  different  manu- 
facturers who  loaned  the  apparatus  used  in  the  investigation  of  the  different  systems 
referred  to  in  this  article. 

115 


A  NEW  RADIATOR  TYPE  OF  HOT-CATHODE  ROENTGEN-RAY  TUBE* 

BY  DR.  W.  D.  COOLIDGE 

INTRODUCTION 

The  type  of  tube  described  in  this  article  was  developed  specifically  for  military  use 
in  the  portable  Roentgen-ray  outfits  at  the  Front.  Its  characteristics  are  such,  how- 
ever, that  it  seems  ultimately  destined  to  supplant  the  earlier  type  of  hot-cathode 
tube  for  all  diagnostic  work. 

THE  PROBLEM 

A  study  of  the  situation  had  shown  that  the  electrical  efficiency  of  existing  portable 
Roentgen-ray  generating  outfits  was  low,1  and  that  it  could  be  very  greatly  increased  if 
a  suitable  tube  could  be  developed  for  operation  directly  from  the  secondary  of  a  high- 
tension  transformer,  without  the  use  of  any  auxiliary  rectifying  device.  As  the  portable 
apparatus  was  intended  for  diagnostic  work,  it  was  highly  desirable  that  the  focal 
spot  in  such  a  tube  should  be  as  small  as  possible  for  handling  the  required  amount  of 
energy. 

THE  STATUS  OF  THE  PRIOR  ART 

The  earlier  form  of  hot-cathode  tube  having  a  solid  tungsten  target  is  capable  of 
rectifying  its  own  current ;  but  only  for  such  amounts  of  energy  as  do  not  heat  the  focal 
spot  to  a  temperature  approximating  that  of  the  cathode  spiral.  As  soon,  however,  as 
any  part  of  the  focal  spot  is  heated  to  a  sufficiently  high  temperature,  it  emits  electrons 
copiously,  and,  therefore,  when  supplied  from  a  source  of  alternating  potential,  it  per- 
mits so-called  "inverse "  current  to  pass.  The  "inverse "  cathode-ray  stream  comes  out 
from  the  focal  spot  in  a  direction  perpendicular  to  the  face  of  the  target  and  proceeds, 
in  the  form  of  a  narrow  pencil,  straight  to  the  glass  wall  of  the  bulb  close  to  and  slightly 
behind  the  cathode.  The  glass  at  this  spot  fluoresces  vigorously,  becomes  locally 
heated,  and  usually  cracks.  As  air  enters  the  bulb,  a  spark  discharge  passes  through 
the  opening  and  it  is  then  easy,  for  one  who  has  not  studied  the  phenomenon,  to  con- 
clude that  the  tube  failed  by  puncturing  under  electrostatic  strain. 

The  local  heating  of  the  glass,  attendant  upon  overloading  a  tube  which  is  running 
on  alternating  current,  can  be  prevented  by  making  the  cathode  focusing  device  of  some 
refractory  metal,  such  as  molybdenum  or  tungsten,  and  so  locating  this  in  the  tube  that 
it  intercepts  the  "inverse"  cathode-ray  stream.2 

While  this  method  has  proved  exceedingly  useful  as  a  safety  device,  when  such  a  tube 
is  to  be  used  on  alternating  current,  it  does  not  appreciably  increase  its  capacity,  for  it 
would  obviously  be  undesirable  to  have  the  cathode  focusing-device  giving  off  Roent- 
gen-rays under  such  bombardment. 

*  From  Genera!  Electric  Review,  Jan.,  1918,  pp.  56-60. 

1  The  electrical  efficiency  of  Roentgen-ray  apparatus  has  usually  been  comparatively  unimportant,  but  as  efficiency  de- 
termines the  weight  and  bulk  of  the  entire  generating  outfit,  it  is  obviousiy  a  very  important  factor  in  the  design  of  portable 
outfits. 

-  Under  these  circumstances,  the  fact  that  the  tubes  being  overloaded  is  shown  by  a  sudden  vigorous  local  heating  of 
the  focusing  device.     In  case  the  area  heated  in  this  way  becomes  sufficiently  hot,  it  becomes  a  third  source  of  cathode  rays. 
These  last  cathode  rays  focus  on  the  target  at  a  point  somewhat  removed  from  the  original  and  legitimate  focal  spot.     This 
can  be  very  easily  observed  and  confirmed  by  the  pinhole-camera  method. 

Copyright,  1919,  by  General  Electric  Review. 

117 


The  essential  condition  to  be  fulfilled  was  that  heat  should  be  more  rapidly  with- 
drawn from  the  focal  spot.  This  could  have  been  accomplished  by  water-cooling,1  but 
this  method  clearly  involved  undesirable  complications  for  portable  work.  Experi- 
ment showed  that  the  most  effective  simple  method  consisted  in  providing  a  target  hav- 
ing a  large  heat  capacity  and  high  heat  conductivity,  and  then  in  arranging  to  effec- 
tively cool  this  mass  of  metal  during  the  interval  between  radiographic  exposures. 
The  importance  of  having  the  target  cold  at  the  start  is  shown  by  the  following  ex- 
perience with  a  tube  having  a  3.2  mm.  focal  spot  and  a  solid  tungsten  target:  With 
the  maximum  allowable  energy  input,  it  was  possible  when  beginning  with  the  target 
at  room  temperature  to  run  four  times  as  long  before  "inverse"  current  appeared  as 
when  the  experiment  was  started  with  the  target  at  dull  red  heat. 

Now,  in  the  case  of  the  ordinary  Roentgen-ray  tube,  the  cooling  of  the  target  from 
dull  red  heat  to  room  temperature  is  an  exceedingly  slow  operation.  The  heat  can  get 
out  only  (1)  by  radiation,  which,  with  small  differences  in  temperature  between  the  hot 
body  and  its  surroundings,  takes  place  at  a  very  low  rate,  and  (2)  by  conduction 
through  the  small  lead-in  wire  and  through  the  glass. 

DESCRIPTION  OF  THE  RADIATOR  TYPE  OF  TUBE 
The  Anode 

The  considerations  which  have  been  described  finally  led  to  the  anode  design  shown 
in  Fig.  1.  The  anode  stem  consists  of  a  solid  bar  of  copper  1.6  cm.  (^g  in.)  in  diameter 


Fig.  1.     Special  Self-rectifying  Roentgen-ray  Tube 

which  is  brought  out  through  the  glass  of  the  anode  arm  to  a  copper  radiator.  The  head 
of  the  anode  consists  of  a  mass  of  specially  purified  copper  which  is  first  cast  in  vacuum 
onto  a  tungsten  button  and  is  then  electrically  welded  to  the  stem.  The  tungsten  but- 
ton, which  is  destined  to  receive  the  cathode  ray  bombardment  is  2.5  mm.  (0.1  in.) 
thick  and  9.5  mm.  (^  in.)  in  diameter. 

The  complete  target,  with  radiator,  weighs  860  gm.  and  has  a  heat  capacity  of  81 
calories  per  degree  centigrade;  while  the  present  standard  solid  tungsten  target,  com- 
plete with  molybdenum  stem  and  iron  supporting  tube,  has  a  heat  capacity  of  less  than 
10  calories.  Because  of  its  greater  heat  capacity,  it  takes  much  longer  to  heat  the 
radiator  type  of  target  to  a  given  temperature  than  it  does  the  solid  tungsten  target. 
What  is  much  more  important,  however,  is  the  fact  that  between  radiographic  ex- 
posures the  target  in  the  new  tube  cools  comparatively  rapidly  owing  to  the  large 
copper  stem  and  the  radiator. 
The  Cathode 

The  cathode  in  this  tube  has  been  designed  to  give  a  focal  spot  3.2  mm.  (^  in.)  in 
diameter  with  a  very  uniform  energy  distribution.2 

1   Coolidge,  Am.  J.,  of  Roentgenology,  pp.  7-8  (1915). 

*  For  the  method  of  developing  a  suitable  cathode  design,  see  American  Journal  of  Roentgenology,  pp.  0-6  (1917). 

118 


Size  of  Bulb 

In  the  standard  hot-cathode  tube  with  the  solid  tungsten  target,  the  target  gets  very 
hot  and  radiates  through  the  glass  walls  of  the  tube  the  greater  part  of  the  energy  it  re- 
ceives. As  a  result,  the  glass  becomes  strongly  heated.  In  the  new  radiator  type  of 
tube,  by  far  the  greatest  part  of  the  energy  imparted  to  the  target  is  conducted  to  the 
radiator.  It,  therefore,  becomes  possible  to  make  the  glass  bulb  very  small.  For 
portable  work,  a  diameter  of  9.5  cm.  (8^4  in.)  has  been  standardized. 

The  Exhausting  of  the  Tube 

The  exhausting  of  a  hot-cathode  tube,  containing  the  anode  which  has  been  de- 
scribed, to  such  a  point  that  the  tube  could  be  sealed  off  from  the  pump  appeared  at 
first  to  be  an  impossibility.  A  point  was  finally  reached  where  after  sealing  the  tube  off 
and  allowing  it  to  stand  over  night,  the  vacuum  was  good.  Upon  operating  such  a 
tube  for  a  few  seconds,  however,  the  vacuum  rapidly  deteriorated  until  the  tube  showed 
a  Geissler  glow.  After  a  second  exhausting,  covering  a  period  of  several  hours,  the  ex- 
perience mentioned  was  duplicated  with  this  difference :  the  tube  operated  successfully 
for  a  somewhat  longer  time.  A  third  exhausting,  again  extending  over  a  period  of  sev- 
eral hours,  finally  resulted  in  a  good  tube.  Since  that  time  it  has  developed  that  the  ex- 
haust is  made  much  easier  by  first  filling  the  tube  with  purified  hydrogen  and  then  heat- 


Fig.  2.     Tube  in  Lead- Glass  Shell 

ing  it  strongly.    It  is  still  a  very  serious  operation,  however,  when  compared  with  even 
the  exhausting  of  the  hot-cathode  tubes  of  the  earlier  type. 

THEORY  OF  OPERATION 

In  the  radiator  type  of  tube,  the  passage  of  "inverse"  current  is  avoided  by  a  con- 
struction which  removes  heat  from  the  focal  spot  so  rapidly  that,  in  normal  use,  it  never 
reaches  the  temperature  at  which  an  appreciable  thermionic  emission  of  electrons  can 
take  place.  In  testing  the  first  tubes  of  this  type,  it  was  found  that  some  other  very 
powerful  cause  was  acting  to  prevent  electron  emission  from  the  focal  spot.  It  de- 
veloped that,  in  this  tube,  the  temperature  of  the  focal  spot  could  be  raised  to  that  of 
the  cathode  spiral  and  even  brought  to  the  melting  point,  while  the  tube  was  operating 
on  alternating  current,  without  having  the  tube  allow  any  appreciable  "inverse"  cur- 
rent to  pass. 

The  most  probable  explanation  of  this  phenomenon  appears  to  be  as  follows :  The 
thermionic  emission  of  electrons  from  a  heated  tungsten  surface  is  very  greatly  re- 
duced by  traces  of  oxygen.1  Now,  in  the  case  of  the  solid  tungsten  target,  the  oxygen 
which  is  originally  present  in  the  metal  is  removed  during  the  exhausting  of  the  tube  by 
maintaining  the  target  for  a  considerable  time  at  intense  white  heat  while  the  tube  is 
connected  to  the  pump.  Under  these  conditions  oxide  of  tungsten  dissociates,  and  the 

1  I.  Langmuir,  Phys.  Rev.,  pp.  465-466  (1913). 

119 


oxygen  is  removed  from  the  tube  by  the  pump.  In  the  radiator  type  of  tube  the  condi- 
tions are  very  different.  There  is,  at  the  beginning  of  the  exhausting  process,  a  large 
amount  of  oxygen  both  in  the  tungsten  button  and  in  the  copper  of  the  target.  During 
the  exhausting,  the  temperature  of  the  target  must  at  all  times  be  kept  below  the  melt- 
ing point  of  copper.  As  a  result,  there  is  probably  sufficient  oxygen  left  in  the  target  to 
account  for  the  observed  phenomenon.  The  oxygen  in  the  tungsten  at  the  focal  spot 
would  be  liberated  by  the  heat  produced  by  cathode  ray  bombardment.  This  oxygen 
would  then  be  chemically  removed  from  the  surrounding  space  by  the  hot  copper  of  the 
target.  Other  oxygen  in  the  metal  layers  just  behind  the  focal  spot  would  then  diffuse 
to  the  focal  spot;  and  this  cycle  of  operations  would  go  on  indefinitely,  the  trace  of 
oxygen  in  the  tungsten  at  the  focal  spot  always  greatly  reducing  the  electron  emission. 
With  a  tube  containing  such  a  target,  heat  is  conducted  away  from  the  focal  spot  so 
rapidly  that  the  tube  could  satisfactorily  be  used  to  do  the  work  for  which  it  was  in- 
tended without  the  help  of  the  phenomenon  mentioned.  The  latter  furnishes  a  factor, 
however,  which  strongly  safeguards  this  type  of  tube  when  rectifying  its  own  current. 

OVERLOAD  LIMITATIONS  AND  BEHAVIOR  OF  THIS  FIRST  MODEL  OF  THE  RADIATOR  TYPE 

TUBE 

The  first  model  of  the  radiator  type  tube  was  designed  to  carry  10  milliamperes  at 
a  5-in.  parallel  spark  between  pointed  electrodes  for  the  time  required  for  making  the 
most  difficult  radiographs,  and  to  carry  a  fluoroscopic  load  of  5  milliamperes  at  a  5-in. 
parallel  spark  continuously  for  an  indefinite  period. 

Under  abuse,  its  behavior  is  as  follows :  When  operated  continuously  with  10  milli- 
amperes at  a  5-in.  parallel  spark  and  with  a  constant  heating  current  in  the  filament 
spiral,  the  milliamperage  may  gradually  drop.  If  the  experiment  is  continued  until  the 
anode  is  red  hot,  and  this  will  take  about  2^  minutes,  the  current  may  drop  to  as  low 
as  6  milliamperes.  (The  amount  of  this  drop  depends  upon  the  degree  of  the  exhaus- 
tion, being  greater  the  poorer  this  is.)  Upon  cooling  the  anode,  the  vacuum  immediately 
returns  to  its  original  condition,  as  shown  by  the  fact  that,  with  the  same  filament 
current,  the  milliamperage  returns  to  10.  It  is  surprising  to  see  how  quick  this  re- 
covery is;  it  has  been  found  to  take  place  even  in  the  short  time  that  it  takes  to  cool  the 
anode  by  holding  the  radiator  under  a  cold-water  faucet.  This  recovery  is  doubtlessly 
due  to  gas  absorption  by  the  anode. 

MEASUREMENT  OF  VOLTAGE  WITH  TUBE  RECTIFYING  ITS  OWN  CURRENT 
Upon  operating  a  tube,  which  rectifies  its  own  current,  directly  from  the  secondary 
of  a  transformer,  the  "inverse"  voltage  is  always  higher  than  the  "useful"  voltage. 
Consequently,  the  measurement  of  tube  voltage  by  a  parallel  spark-gap  used  in  the  or- 
dinary way  would,  in  general,  be  very  misleading,  for  the  observed  spark  length  cor- 
responds to  the  "inverse"  voltage  and  not  to  that  which  produces  the  Roentgen-rays 
and  hence  determines  their  nature.  A  simple  and  very  satisfactory  method  of  dealing 
with  this  difficulty  consists  in  connecting  an  alternating-current  voltmeter  across  the  pri- 
mary of  the  transformer  and  then  calibrating  once  for  all  this  combination  of  transformer 
and  voltmeter.  The  calibration  can  be  conveniently  made  with  the  help  of  a  kenotron 
connected  in  series  with  the  Roentgen-ray  tube.  The  two  tubes  should  be  so  connected 
that  current  can  pass  through  the  Roentgen -ray  tube  in  the  right  direction.  If  now  a 
spark-gap  is  connected  across  the  Roentgen-ray  tube  terminals,  it  measures  the  useful 
voltage  (and  this  is  essentially  what  it  would  be  if  the  kenotron  were  not  in  the  circuit, 
for  the  voltage  drop  in  a  suitable  kenotron  is  not  more  than  one  or  two  hundred  volts 
and  can  hence  be  neglected) .  If  the  spark-gap  were  connected  directly  across  the  trans- 

120 


former  terminals  it  would  measure  the  "inverse"  voltage.  The  difference  between  the 
two  will  depend  upon  the  load  and  hence  it  is  necessary  that  the  calibration  should  be 
made  for  every  load  which  it  is  desired  to  use  with  the  tube.  Unless  appreciable 
changes  take  place  in  the  wave  form  of  the  current  supply,  a  single  calibration  suffices, 
and  this  can  be  made  by  the  manufacturer  of  the  transformer.  Unless  otherwise  speci- 
fied, the  voltage  referred  to  in  this  article  is  always  the  "useful "  and  not  the  "inverse " 
voltage. 

ATTACHMENT  OF  RADIATOR  TO  TARGET  STEM 

In  the  first  radiator  type  tubes  made,  the  radiator  was  soft-soldered  to  the  copper 
stem  of  the  target.  It  was  later  found  that  the  tube  would  stand  almost  as  hard  abuse 
if  the  solder  was  omitted  and  the  radiator  was  simply  slipped  on  over  the  stem  and  held 
in  place  by  a  thumb-screw.  This  has  made  it  possible,  for  some  applications,  to  adapt  a 
simple,  close-fitting,  two-part,  lead-glass  shield  to  the  tube. 

LEAD-GLASS  PROTECTIVE  SHIELD 

As  such  a  shield  is,  in  some  cases,  a  matter  of  considerable  importance,  it  is  perhaps 
well  to  discuss  briefly  a  form  which  seems  very  satisfactory.  It  is  evident  that  there  will 
be  some  applications  where  the  tube  shield  ought,  for  a  given  amount  of  protection,  to 
be  as  light  as  possible.  In  these  cases,  it  should  obviously  have  the  same  form  as  the 
tube  and  should  fit  closely  to  the  latter.  Furthermore,  it  should  let  the  heat  energy  of 
the  cathode-spiral  radiate  through  it,  as  otherwise  both  the  shield  and  the  tube  would, 
with  prolonged  filament  excitation,  get  very  hot.  The  ideal  light-weight  shield  will  then 
be  a  non-conductor  of  electricity,  to  permit  of  its  being  closely  fitted  to  the  tube,  and 
will  be  transparent  to  ordinary  light  radiations.  Glass  containing  sufficient  lead  would 
be  a  satisfactory  material  from  which  to  make  such  a  shield.  The  ordinary  X-ray 
protective  glass  with  which  the  author  is  familiar  does  not  have  as  high  a  lead  content 
as  seems  desirable.  The  best  samples  which  he  has  tested  have  to  be  used  in  layers  10  to 
1 2  times  as  thick  as  metallic  lead  to  give  the  same  protection  as  the  latter.  Experiments 
made  by  Dr.  G.  Stanley  Meikle,  of  this  laboratory  showed  that  glass  could  be  made  ex- 
perimentally which  contained  so  much  lead  that,  for  the  same  protective  effect,  the 
glass  layer  had  to  be  only  1.4  times  as  thick  as  sheet  lead.  Such  glass  when  melted  at- 
tacls  s  the  glass-pot  quite  readily  and  may,  therefore,  be  difficult  to  manufacture.  How- 
ever, the  author  has  recently  been  able  to  obtain  glass*  containing  enough  lead  so  that, 
for  the  same  protective  effect,  the  glass  layer  has  to  be  but  4  times  as  thick  as  sheet 
lead. 

The  properties  of  this  glass  are  such  that  it  cannot  be  readily  blown  into  thick- 
v\  ailed  bulbs,  but  it  can  be  pressed  in  moulds.  Fig.  2  shows  a  picture  of  a  tube  sur- 
rounded by  such  a  shield  which  is  made  in  two  parts  bolted  together  in  the  middle.  A 
hole  in  one  side  permits  egress  to  the  desired  bundle  of  Roentgen  rays. 

APPLICATIONS  OF  THE  TUBE 

Two  portable  Roentgen-ray  generating  units  have  already  been  built  around  the 
first  model  of  this  tube.  These  are  the  "U.S.  Army  Portable  Unit"  and  the  "U.S. 
Army  Bedside  Unit."  In  both  of  these  outfits  the  tube  is  operated  directly  from  a 
high-voltage  transformer  with  no  auxiliary  rectifying  device. 

This  model  of  the  tube  should  also  be  useful  for  general  fluoroscopic  work  and  for  all 
radiographic  work  which  can  be  done  with  a  focal  spot  as  small  as  3.2mm.  (i/g-in.)  in 


*  From  the  Corning  Glass  Works. 

121 


diameter.     (This  means  an  amount  of  energy  not  in  excess  of  that  corresponding  to  10 
m.a.  at  a  5-in.  parallel  spark  between  pointed  electrodes.) 

For  more  rapid  radiography,  other  models  will  be  developed  with  larger  focal  spots, 
but  probably  with  the  same  external  tube  dimensions. 

ADVANTAGES  OF  THE  RADIATOR  TYPE  TUBE 

The  advantages  which  the  new  type  of  tube  possesses  over  the  earlier  type  for  diag- 
nostic work  are  the  following : 

1 .  It  can  be  used  to  rectify  its  own  current  under  conditions  of  service  which  are 
much  severer  than  would  be  permissible  with  the  earlier  type  of  hot-cathode  tube  having 
the  same  size  of  focal  spot. 

2.  The  bulb  can  be  smaller  than  is  permissible  with  the  earlier  type  handling  the 
same  amount  of  energy. 

3.  On  either  alternating-  or  rectified-current  it  will  carry  the  maximum  allowable 
energy  for  a  much  longer  time. 

4.  It  can  have,  even  for  heavy  duty,  a  close-fitting  tube  shield. 

The  author  desires  to  express  his  thanks  to  George  Hotaling,  C.  N.  Moore,  and 
Leonard  Dempster  for  their  assistance  in  carrying  out  the  work  which  has  been 
described. 


122 


USE  AND  ABUSE  OF  ROENTGEN-RAY  TUBES  * 

BY  C.  N.  MOORE 

I  feel  rather  as  though  an  apology  were  due  for  appearing  before  a  body  of  trained 
roentgenologists  to  speak  upon  the  subject  of  the  use  and  abuse  of  roentgen-ray  tubes. 
My  only  excuse  for  doing  so,  is  the  belief  that  experience  gained  in  the  development, 
manufacture  and  testing  of  tubes,  together  with  the  examination  of  tubes  returned  for 
repairs,  may  enable  me  to  be  of  some  service  by  pointing  out  the  conditions  under  which 
tubes  were  designed  to  operate.  I  will  mention  some  of  their  limitations,  and  by  means 
of  sample  tubes  and  a  few  simple  experiments  show  some  of  the  results  of  abuse.  In  the 
present  emergency,  it  is  highly  desirable  to  conserve  and  utilize  to  the  utmost  degree  the 
tubes  which  must  be  transported  under  such  difficulties  and  for  such  distances.  I  hope 
that  nothing  I  shall  say  will  conflict  with  what  you  are  being  taught  in  the  various 
schools  for  military  roentgenology.  My  remarks  will  apply  specifically  to  the  hot 
cathode  type  of  tube,  but  will  apply  in  the  main  to  all  types. 

Turning  first  to  the  all-tungsten  target  tube  in  the  seven-inch  bulb — a  tube  de- 
signed for  diagnostic  work,  both  roentgenographic  and  fluoroscopic,  and  for  roentgen- 
otherapy .  It  is  intended  to  be  operated  under  the  following  conditions :  ( 1 )  With  volt- 
ages as  high  as  that  corresponding  to  a  ten-inch  parallel  spark  gap  between  points,  and 
with  currents  depending  upon  conditions  to  be  explained  later,  and  (2)  upon  apparatus 
delivering  rectified  current  to  the  tube.  You  are  all  perfectly  familiar  with  the  operation 
of  this  tube,  but  I  wish  to  say  a  few  words  regarding  the  conditions  just  mentioned. 

First,  in  regard  to  the  question  of  the  amount  of  energy  that  a  tube  will  carry.  In 
this  case,  energy  may  be  considered  as  the  product  of  the  milliamperage  times  the 
potential  (spark  gap).  Once  the  target  design  is  fixed,  the  allowable  energy  input  is 
determined  principally  by  four  things:  (1)  The  target  material,  (2)  the  area  of  the 
focal  spot,  (3)  the  time  during  which  the  energy  is  applied,  and  (4)  the  temperature  of 
the  target  at  the  time  the  energy  is  applied.  Of  the  energy  applied  to  the  focal  spot, 
approximately  99.8  per  cent  is  converted  into  heat,  so  that  the  question  of  the  allowable 
energy  input  is  really  a  question  of  the  rapidity  with  which  heat  can  be  removed  from 
the  focal  spot  and  of  the  melting  point  of  the  target  material.  The  metal  tungsten  is 
used  today  for  the  target  face  in  practically  all  tubes.  It  has  a  melting  point  of  3300 
deg.  C.  If  this  temperature  is  exceeded,  tungsten  will  melt.  The  filament  in  this 
incandescent  lamp  is  made  of  tungsten.  I  am  going  to  raise  its  temperature  above 
3300  deg.  C.  by  operating  it  at  a  voltage  considerably  in  excess  of  that  for  which  it  was 
built.  .As  I  cut  out  resistance  in  series  with  the  lamp,  the  tungsten  becomes  hotter  and 
hotter,  until  finally  at  this  point  where  it  is  white  hot,  it  melts.  Probably  the  worst 
abuse  of  the  roentgen-ray  tube  is  due  to  a  lack  of  consideration  of  the  fact  that  tungsten 
has  this  definite  melting  point.  Present  practice  is  resulting  in  repeated  melting  of  the 
focal  spot  in  many  tubes.  The  molten  tungsten  vaporizes  rapidly  in  an  evacuated 
space,  and  deposits  in  a  thin  film  over  the  active  hemisphere,  as  is  shown  by  this  tube. 
This  mirror-like  metal  deposit  on  the  inside  of  the  glass  should  not  be.  confused  with  the 
violet  coloration  which  always  results  when  the  particular  kinds  of  glass  used  in  these 
bulbs  are  subjected  to  prolonged  exposure  to  roentgen  rays.  This  violet  color  is  due  to 
some  change  taking  place  within  the  glass  itself  and  is  perfectly  harmless.  The  thin  film 
of  tungsten  exerts  no  appreciable  filtering  effect  upon  the  roentgen  ray,  but  it  does  disturb 


*  Read  before  the  Nineteenth  Annual  Meeting  of  the  American  Roentgen  Ray  Society,  Chattanooga,  Tenn.,  September, 
1918.  Reprinted  from  the  American  Journal  of  Roentgenology,  November,  1918,  Vol.  V,  No,  II,  pp.  529-531.  Copyright, 
1918. 

123 


the  electrical  conditions  within  the  tube.  Experiments  in  the  laboratory  have  shown 
that  a  grounded  metal  wire  can  be  brought  up  into  contact  with  a  clear  bulb  when  the 
tube  is  operating,  whereas  with  a  tube  having  a  metal  deposit  inside,  such  a  wire  must 
be  moved  away  a  number  of  inches  to  avoid  puncturing  the  tube.  The  metal  of  the 
tube  stand  is  usually  grounded  and  the  large  number  of  punctured  bulbs  similar  to  the 
one  which  I  am  showing  you,  would  be  greatly  reduced  if  the  practice  of  blackening 
bulbs  were  discontinued.  Furthermore,  I  want  to  call  your  attention  to  the  fact  that 
this  practice  results  in  no  appreciable  gain.  You  may  succeed  in  increasing  your 
roentgen  ray  production  a  few  per  cent  thereby,  but  this  is  hardly  of  interest.  At  the 
same  time  you  have  probably  greatly  reduced  the  life  of  your  tube.  Properly  used,  a 
tube  will  gradually  acquire  the  deep  violet  color  shown  in  this  tube,  which  was  returned 
for  a  trifling  mechanical  defect  after  two  and  a  half  years  of  constant  use.  If  used  in  the 
same  conservative  manner,  this  tube  should  last  many  years  more. 

Just  a  few  words  in  regard  to  the  other  factors  involved  in  the  question  of  allowable 
energy  input.  It  is  obvious  that  more  energy  may  be  applied  to  a  large  focal  spot  than 
to  a  small  one  without  exceeding  the  allowable  temperature.  For  a  given  size  of  focal 
spot,  more  energy  may  be  applied  for  a  short  time  than  would  be  possible  if  the  time  of 
application  were  longer.  And  finally,  for  a  given  size  of  focal  spot,  more  energy  may  be 
applied  to  a  cold  target  without  melting  the  focal  spot  than  to  one  that  is  already  hot, 
because  in  the  case  of  the  cold  target,  heat  will  flow  away  from  the  focal  spot  more 
rapidly. 

.  What  I  have  said  so  far  has  applied  mainly  to  roentgenographic  work.  For  con- 
tinuous operations  in  roentgenotherapy,  the  limiting  feature  is  not  so  much  the  target 
as  the  glass  of  the  bulb.  It  is  possible  to  get  the  body  of  the  target  so  hot  that  heat 
radiated  from  it  may  cause  gas  to  be  driven  out  of  the  glass  or  may  even  melt  the  glass. 
Adequate  cooling  of  the  bulb  is  necessary  to  prevent  this. 

I  have  already  stated  that  the  present  seven-inch  tube  with  the  solid  tungsten  tar- 
get was  designed  to  operate  upon  apparatus  delivering  rectified  current  to  the  tube. 
It  is  not  designed  to  operate  directly  from  the  terminals  of  a  high  tension  transformer 
without  the  interposition  of  a  suitable  rectifying  device,  or  from  the  terminals  of  a  coil 
without  the  use  of  valve  tubes.    Operation  under  these  conditions  constitutes  one  of  the 
worst  abuses  with  which  we  are  familiar.    Again  it  is  a  question  of  the  temperature  of 
the  focal  spot.     Tungsten  at  a  temperature  of  about  2000  deg.  C.  begins  to  emit 
electrons  at  an  appreciable  rate.    As  a  consequence,  if  a  source  of  alternating  potential 
is  applied  to  the  terminals  of  a  tube  with  the  focal  spot  or  any  point  in  the  focal  spot,  at 
or  above  this  temperature,  inverse  current  will  pass  through  the  tube.    This  inverse 
I  current  may  focus  on  the  glass  in  the  cathode  stem,  causing  cracking  and  the  emission 
I.  of  gas,  or  it  may  focus  on  the  glass  of  the  bulb  directly  below  the  cathode.    It  is  cus- 
/  ternary  to  speak  of  a  tube  being  punctured  by  inverse  at  this  point.    What  really  hap- 
/   pens  is  that  the  inverse  current  heats  the  glass  locally  so  that  it  actually  melts,  or  else 
/    sets  up  strains  so  that  the  glass  cracks  during  this  or  some  subsequent  operation.  After 
/     cracking,  air  rushes  in  and  the  electric  current  traveling  along  the  air  current  gives  the 
I,    impression  that  a  spark  was  the  cause  of  the  puncture.    The  presence  of  inverse  is  al- 
ways indicated  by  green  fluorescence  at  the  cathode  end  of  the  tube.    I  am  now  going 
to  do  something  which  should  never  be  done.  I  am  going  to  run  this  tube  directly  from 
the  terminals  of  a  transformer  with  no  other  rectifying  device.    Within  a  few  seconds 
from  the  start  you  will  see  green  fluorescence  appearing  in  the  cathode  arm.    Now  that 
the  target  is  at  a  dull  red  temperature,  the  focal  spot  is  hotter  and  you  see  the  green 
fluoresence  immediately  upon  closing  the  switch.    The  tube  has  not  yet  been  injured 


but  if  I  should  continue  as  I  am  about  to  do  now,  I  shall  undoubtedly  ruin  the  tube. 
You  see  the  inverse  current  focusing  now  on  the  glass  below  the  cathode  and  a  moment 
later  so  much  gas  has  been  evolved  that  I  shall  have  to  discontinue  operation  of  the 
tube.  Subsequent  examination  of  the  tube  shows  that,  while  cooling,  the  glass  has 
cracked  at  the  point  where  the  inverse  current  focused  upon  it.  The  same  thing  would 
happen  on  an  induction  coil.  It  will  also  occur  on  the  regular  interrupterless  machines 
if  the  rectifying  switch  is  not  set  properly. 

Turning  now  to  the  radiator  type  of  tube  which  was  developed  for  use  in  connection 
with  the  portable  army  outfits.  This  tube  is  adapted  to  diagnostic  use  only,  and  the 
allowable  energy  input  is  limited  to  that  corresponding  to  10  ma.  at  a  five-inch  parallel 
spark  gap  For  continuous  operation  in  fluoroscopy,  the  current  is  limited  to  5  ma.  It 
is  not  suitable  for  therapy  for  the  reason  that  it  has  not  been  designed  to  operate  at  the 
voltages  used  in  such  work  and  because  it  will  not  carry  continuously  more  than  one 
quarter  of  a  kilowatt.  The  seven-inch  tube  with  no  copper  in  the  anode  will  carry  con- 
tinuously about  one  kilowatt. 

The  radiator  type  tube  is  so  designed  that  it  will  rectify  its  own  current  and  can 
therefore  be  operated  directly  from  the  terminals  of  a  high  tension  transformer  or  a  coil. 
You  are  all  familiar  with  the  manner  in  which  it  is  used  on  the  portable  outfits.  It  will 
operate  equally  well  on  outfits  delivering  rectified  current  if  the  limits  mentioned  above 
are  not  exceeded. 

I  am  now  going  to  run  a  radiator  tube  on  this  apparatus  taken  from  a  portable  out- 
fit, with  a  current  of  10  ma.  to  demonstrate  the  behavior  of  this  tube  under  abuse.  The 
longest  roentgenographic  exposure  is  probably  not  over  forty  seconds.  You  will  ob- 
serve from  the  milliammeter  that  the  tube  has  remained  fairly  constant  during  this 
time.  If,  now,  I  continue  running  longer  without  adjusting  the  filament  current,  the 
milliamperage  will  gradually  drop  until  at  the  end  of  two  minutes  it  has  reached  8  ma. 
This  falling  off  in  the  milliamperage  is  due  to  the  presence  of  gas  driven  out  of  the  copper 
in  the  anode  by  heat.  If  I  set  the  tube  aside,  the  copper  as  it  cools  will  reabsorb  the  gas 
and  the  tube  will  return  to  its  original  condition.  Because  of  lack  of  time,  however,  I 
shall  have  to  omit  this  part  of  the  experiment.  1  am  now  going  to  abuse  this  tube. 
I  will  operate  it  at  10  ma.  until  it  shows  signs  of  distress.  At  the  end  of  about  four 
minutes  the  copper  of  the  target  has  reached  such  a  temperature  that  I  shall  have  to 
discontinue  to  avoid  melting  it.  Frequently  before  this  point  is  reached,  the  tube  will 
show  green  fluorescence  throughout  the  bulb,  owing  to  the  fact  that  so  much  gas  will 
have  been  evolved  that  further  operation  is  unsafe. 


125 


THE  RADIATOR  TYPE  OF  TUBE* 

BY  W.  D.  COOLIDGE 

Research  Laboratory,  General  Electric  Company 
SCHEXECTADY,  N.  Y. 

The  radiator  type  of  tube  has  already  been  described.1  Up  to  the  present  time,  its 
use  has  been  confined  exclusively  to  the  army  and  in  this  service  it  has  been  used  as  a 
self-rectifying  tube  in  the  Portable  and  Bedside  outfits.  Owing  to  the  fact  that  the 
tube  has  recently  become  generally  available  for  use  on  existing  standard  generating 
outfits,  the  following  amplification  of  what  has  been  published  will  perhaps  be  useful. 

The  leading  motive  in  the  development  of  this  type  was  originally  to  get,  in  the 
form  of  a  small  simple  and  rugged  structure,  a  tube  which  would  operate  satisfactorily 
when  alternating  current  was  supplied  to  its  terminals.  As  will  be  seen  from  the  follow- 
ing, however,  the  use  of  the  tube  should  not  be  limited  to  such  service,2  for  it  will  do 
better  diagnostic  work  on  any  generating  outfit  than  will  the  earlier  form  of  hot- 
cathode  tube  with  the  solid  tungsten  target. 

FIELD  OF  USEFULNESS  OF  THE  RADIATOR  TUBE 

This  type  of  tube  makes  possible  the  use  of  a  simple  transformer3  as  a  current  gen- 
erating outfit  (for  direct  current  circuits  a  converter  must  be  added).  It  has  been 
found  that  a  little  transformer  of  suitable  design  and  weighing  but  slightly  over  50 
pounds  is  adequate  for  supplying  a  current  of  50  milliamperes  at  a  useful  gap  of  5-in.  to 
a  self -rectifying  tube.  This,  of  course,  means  that  a  very  small  and  relatively  inexpen- 
sive outfit  of  this  type  can  be  made  to  do  even  very  rapid  diagnostic  work.  The  free- 
dom from  mechanical  and  electrical  complications  and  from  noise  and  the  diminution  in 
the  odors  attendant  upon  high  tension  discharges  in  air  seem  very  much  in  favor  of  this 
system. 

The  tube  operates  satisfactorily  on  the  transformer,  the  interrupterless  outfit,  and 
the  induction  coil,  and  is  hence  generally  applicable  to  diagnostic  work.  With  the  in- 
duction coil  it  renders  a  valve  tube  unnecessary.  With  the  interrupterless  machine,  it 
offers  the  advantage  that  it  is  not  injured  by  a  faulty  setting  of  the  mechanical  rectifier. 
With  every  current  source  it  permits  of  the  intermittent  use  of  more  energy  than  could 
in  practice  safely  be  carried  by  a  tube  with  a  solid  tungsten  target  and  the  same  size  of 
focal  spot. 

It  is  not,  in  its  present  form,  adapted  to  therapeutic  work,  for,  with  continuous 
operation,  it  will  carry  less  than  one  fourth  as  much  energy  as  the  7-inch  hot-cathode 
tube  with  solid  tungsten  target. 

CURRENT-CARRYING  CAPACITY  OF  THE  RADIATOR  TUBE 

For  a  given  voltage,  the  current-carrying  capacity  of  the  radiator  type  of  tube  is  a 
very  definite  quantity.  This  is  due  to  the  fact  that  the  target  of  the  radiator  tube  cools 
so  rapidly,  between  exposures,  that  every  exposure  is  started  with  a  relatively  cold  tar- 
get. What  can  be  done  with  the  tube  then,  in  radiographic  work,  depends  but  little  on 
its  past  history. 

This  will  perhaps  be  made  clearer  by  the  following  consideration :  The  target,  with 
its  heat-storage  capacity,  is  like  a  reservoir  used  for  the  storage  of  water.  It  is  capable 

*  Copyright    1919   by  American  Journal  of  Roentgenology. 

1  Gen.  Elec.  Rev.,  21,  pp.  55-60  (1918). 

2  It  is,  perhaps,  necessary  to  place  additional  emphasis  on  this  point  for  the  reason  that  one  is  instructed  in  the  U.  S. 
Army  X-ray  Manual  not  to  use  this  tube  on  large  installations  or  interrupterless  machines.    This  was  doubtless,  under  the 
circumstances  existing  at  the  time,  a  wise  regulation.    To  apply  this  regulation  to  civil  practice,  however,  would  be  equiva- 
lent to  saying  that  no  10-ma.  X-ray  tube  should  be  operated  from  a  large  machine. 

3  This  word,  transformer,  is  so  generally  incorrectly  used  by  the  roentgenologist  that  a  definition  will  not  be  out  of  place. 
A  transformer  is  a  device  for  transforming  an  alternating  current  of  one  voltage  to  that  of  another.    In  its  simplest  form,  it 
consists  of  two  coils  of  wire  placed  around  a  common  iron  core.     It  is  not  an  "  Interrupterless  Machine  " — the  latter  consists 
essentially  of  a  transformer,  a  synchronous  motor,  and  a  mechanical  rectifying  switch. 

127 


of  absorbing  and  storing  up  an  amount  of  heat  energy,  which  is  determined  by  the  size 
of  the  target  and  by  the  amount  of  heat  already  in  it,  just  as  the  reservoir  is  capable  of 
taking  up  an  amount  of  water  determined  by  its  size  and  by  the  amount  of  water  al- 
ready in  it.  The  capacity  of  the  target  to  take  up  heat  is  always  the  same  provided  it  is 
always  at  the  same  temperature  at  the  beginning  of  the  exposure. 

It  is  then  a  simple  matter  for  the  manufacturer  to  specify  the  capacity  of  tubes  of 
the  radiator  type.4 

In  the  particular  service  for  which  the  radiator  tube  was  first  developed,  the  amount 
of  electrical  energy  available  at  the  tube  terminals  was  strictly  limited  and  corresponded 
to  10  ma.  at  a  5-inch  useful  gap.  The  focal  spot  in  the  first  model  was  then  made  as 
small  as  could  be  conservatively  used  with  this  amount  of  energy  (i/g-inch  in  diameter). 
In  this  connection,  the  writer  wishes  to  lay  emphasis  on  the  word  conservatively.  He  has 
seen  one  of  these  tubes  which  had  been  in  almost  constant  radiographic  service  in  a  base 
hospital  for  a  period  of  several  months.  It  had  been  operated  on  an  interrupterless 
machine  with  a  load  of  10  ma.  at  a  5-inch  gap.  Inspection  showed  that  the  focal-spot 
had  never  even  been  frosted  and  could  not  be  identified  without  operating  the  tube. 
Under  such  circumstances  and  with  such  consistent  use,  there  is,  of  course,  a  temptation 
on  the  part  of  the  operator  to  increase  the  load  and  to  exceed  the  current  specified  by 
the  manufacturer.  The  writer,  however,  cannot  see  a  justification  for  doing  this.  If, 
without  serious  damage,  a  current  of  say  30  milliamperes  could  occasionally  be  used 
with  a  tube  marked  10  ma.,  it  might  seem  worth  while;  but  it  can't  be  done.  It  will 
ruin  the  tube.  With  the  tube,  in  the  base  hospital  in  question,  the  current  could  un- 
doubtedly have  been  raised  50  per  cent  and  this  would  have  reduced  the  time  of  ex- 
posure to  two  thirds  of  what  it  had  been.  The  point  which  the  writer  wishes  to  make, 
however,  is  that  the  gain  derived  from  attempting  to  operate  a  radiator  tube,  or  any 
other  roentgen  ray  tube  of  any  kind,  or  a  jack-knife,  or  any  other  device,  at  the  break- 
down point  is  out  of  all  proportion  to  the  cost. 

Later  in  the  war  it  was  felt  by  some  that  there  was  need,  in  the  army  service,  of  a 
limited  number  ot  mobile  outfits  having  a  higher  current  capacity  than  the  standard 
U.  S.  Army  Portable  outfit.  For  this  reason,  experimental  work  was  undertaken  on  a 
larger  size,  3  kw.,  Delco-light  set.  It  developed  that,  with  certain  minor  changes, 
this  set,  when  connected  to  the  small  X-ray  transformer  used  in  the  standard  Portable 
outfit,  was  capable  of  delivering  30  ma.  at  a  5-inch  useful  gap  to  a  self-rectifying  tube. 

It  was  found  that  a  radiator  type  of  tube  with  a  ^-inch  focal  spot  behaved  nicely 
with  the  30  milliampere  load.  Except  for  the  larger  focal  spot,  this  tube  was  identical 
with  the  10  milliampere  radiator  tube. 

There  is  every  reason  to  think  that,  with  a  still  larger  focal  spot,  this  same  design  of 
radiator  tube  will  be  found  equally  satisfactory  for  100  milliamperes  or  more,  and  that 
it  will  still  operate  just  as  well  on  alternating  as  on  rectified  current. 

The  current-carrying  capacity  is  the  same  for  alternating  as  it  is  for  rectified  current, 
as  the  tube  will  safely  rectify  any  current  which  does  not  damage  the  focal  spot. 

OPERATION  ON  TRANSFORMER  (WITHOUT  MECHANICAL  OR  OTHER  RECTIFIER) 
When  used  where  it  has  to  rectify  its  own  current,  the  radiator  type  of  tube  shows 
its  greatest  superiority  over  the  hot  cathode  tube  with  the  solid  tungsten  target,5  used 
under  the  same  conditions.    This  is  illustrated  by  the  following  experimental  data: 

4  In  the  case  of  the  earlier  type  of  tube  wich  the  solid  tungsten  target,  the  specification  of  current-carrying  capacity- 
was  always  rather  unsatisfactory  for  the  reason  that  the  cooling  of  the  target  in  this  tube,  between  exposures,  was  rela- 
tively very  slow.      Because  of  this  latter  fact,  the  target  temperature,  at  the  beginning  of  an  exposure  might  be  anything 
between  room  temperature  and   intense  white  heat.    The  amount  of  heat  which  the  solid  tungsten  target  was  capable  of 
storing  up  and,  hence,  the  current-carrying  capacity,  was  then  a  very  variable  quantity,  depending  on  the  temperature 
of  the  target  at  the  beginning  of  the  exposure. 

5  The  latter  should  never  be  used  to  rectify  its  own  current.      The  reasons  will  be  obvious  from  the  following  data. 

128 


30-MILLIAMPERE  RADIATOR  TUBE,  OPERATED  FROM  TRANSFORMER 
(WITHOUT  RECTIFIER) 

With  the  target  at  room  temperature,  a  current  of  30  milliamperes  at  a  5-inch  useful 
gap  was  applied  continuously  for  35  seconds.  At  the  end  of  this  time,  there  was  a  slight 
flash  of  green  fluorescence  in  the  tube  and  evidence  of  a  slight  high  voltage  surge  on  the 
line.  The  target  was  bright  red.  No  harm  had  been  done  to  the  tube,  but  it  would  have 
been  unsafe  to  continue  without  interruption. 

With  intermittent  operation,  it  was  found  that  a  6-second  exposure  with  30  milli- 
amperes at  a  5-inch  useful  gap  could  be  repeated  indefinitely  with  40  second  intervals 
between  exposures. 

With  suitable  intervals  between,  exposures  with  30  ma.  ranging  in  time  from  0  to  35 
seconds  could  then  be  made. 

MEDIUM  Focus  T-INCH  TUBE  WITH  SOLID  TUNGSTEN  TARGET,  OPERATED  FROM 

TRANSFORMER  (WITHOUT  RECTIFER) 

Starting  with  the  target  at  room  temperature,  this  tube  was  operated  with  30  milli- 
amperes at  a  5-inch  useful  gap  for  10  seconds.  At  the  end  of  this  time  it  showed  green 
fluorescence  at  the  cathode  end,  indicating  that  it  was  beginning  to  pass  inverse  current. 
The  target  was  red  hot.  Experience  has  shown  that  had  the  operation  of  this  tube  been 
continued  further  without  interruption,  it  would  have  been  put  permanently  out  of 
commission,  either  from  cracking  the  bulb  back  of  the  cathode  or  from  deterioration 
of  vacuum. 

After  waiting  15  minutes  for  the  target  to  cool,  it  was  found  that  the  tube  would 
carry  the  30  ma.  load  for  5^2  seconds,  at  the  end  of  which  time,  inverse  again  appeared. 
After  a  further  wait  of  21  minutes,  it  was  possible  to  operate  it  for  7  seconds 

This  tube  then  would  have  stood  from  a  cold  start,  a  single  30  ma.  exposure  of  10 
seconds,  and  repeated  exposures  of  6  seconds  duration  with  intervals  of  about  20  min- 
utes between. 

BROAD-FOCUS    T-INCH    TUBE    WITH    SOLID    TUNGSTEN   TARGET,    OPERATED    FROM 

TRANSFORMER  (WITHOUT  RECTIFIER) 

Experiment  showed  that  this  tube  could  be  run  with  30  milliamperes  at  a  5-inch 
gap  for  6-second  exposures  at  intervals  of  15  minutes 

FURTHER  CONSIDERATIONS 

With  the  present  design,  the  7-inch  tubes  with  solid  tungsten  target  show  con- 
siderable differences  in  the  evenness  of  distribution  of  energy  over  the  focal  spot.  The 
two  7-inch  tubes  tested  were  taken  at  random  and  for  this  icason  they  do  not  represent 
the  worst  conditions.  Other  tubes  will  be  found  which  will  behave  even  worse  than 
these  when  operating  on  a1ternating  current. 

It  is  also  a  fact  that  commercial  experience  with  this  type  of  tube  operating  direct- 
ly from  a  transformer  (without  a  rectifying  device)  has  resulted  in  the  rapid  destruc- 
tion of  tubes  and  has  been  thoroughly  unsatisfactory 

The  30  milliampere  radiator  tube  was  designed  to  rectify  its  own  current.  It  was, 
in  the  above  mentioned  experiments,  always  much  further  from  the  actual  breakdown 
point  than  were  the  other  tubes.  A  load  much  higher  than  that  used  would  have  been 
necessary  to  produce  inverse  with  the  radiator  tube  (the  fluorescence  referred  to  in  the 
test  was  due  to  gas  given  off  from  the  hot  copper  and  was  not  a  manifestation  of  inverse 
current),  and,  had  inverse  been  produced,  it  would  have  been  intercepted  by  the 
hemispherical  cathode  and  so  kept  from  hitting  the  glass. 

129 


The  above  tests  can  then  be  summarized  as  follows :  Running  directly  from  a  trans- 
former (without  a  mechanical  rectifying  switch),  the  30-milliampere  radiator  tube  was 
operated  for  single  periods  of  35  seconds  duration,  while  the  limiting  time  of  operation 
with  the  medium  focus  solid  tungsten  target  tube  was  10  seconds  Six-second  periods 
of  operation  could  be  repeated  indefinitely  with  40-second  intervals  with  the  radiator 
tube,  while  intervals  of  15  and  20  minutes  were  needed  with  the  other  two  tubes. 
Furthermore,  the  radiator  tube  was  not  endangered  by  the  above  tests,  while  the  others 
were  being  operated  very  close  to  the  breakdown  point. 

Attention  should  be  called  to  the  fact  that  the  above  tests  were  forced  tests.  They 
are  given  merely  to  show  the  radically  different  behavior  of  the  two  types  of  tube  when 
operating  on  alternating  current.  At  the  present  time,  there  is  certainly  no  occasion  in 
diagnostic  work  to  use  30  ma.  at  a  5-inch  useful  gap  for  a  period  of  anything  like  35 
seconds  or  to  make  any  considerable  number  of  consecutive  6-second  exposures  with 
a  time  interval  of  only  40  seconds  between  them. 

OPERATION  ON  INTERRUPTERLESS  MACHINE 

For  satisfactory  diagnostic  work,  it  is  imperative  that  a  voltage  control  of  the  auto- 
transformer  type  rather  than  the  resistance  type  be  employed.  The  use  of  auto-trans- 
former control  will  ensure  constant  voltage  and  will  hence  greatly  facilitate  the  duplica- 
tion of  results. 

The  results  obtained  should  be  better  than  with  the  tube  having  a  solid  tungsten 
target;  for  in  general,  for  a  given  amount  of  energy,  a  smaller  focal  spot  can  be  used  in 
the  radiator  tube. 

The  tube  also  offers  this  additional  advantage,  for  use  on  the  above  outfit,  that  a 
faulty  setting  of  the  rectifying  device,  resulting  perhaps  in  sufficient  inverse  to  ruin 
the  tube  with  the  solid  tungsten  target,  will  not  injure  the  radiator  tube. 

OPERATION  ON  INDUCTION  COIL 

The  radiator  tube  is  well  adapted  to  induction  coil  use  and  renders  a  valve  tube  un- 
necessary. The  cathode  filament  may  be  supplied  with  current  from  an  insulated 
storage  battery  or  from  a  small  converter  and  special  step-down  transformer.  For 
diagnostic  work,  however,  the  induction  coil  outfit,  because  of  its  complications,  does 
not  seem  competitive  with  the  simple  transformerr. 

USE  OF  LOW  MlLLIAMPERAGE 

Some  enthusiastic  users  of  a  10  ma.  tube  seem  to  feel  that  there  is  some  mysterious 
advantage  derived  from  the  use  of  low  milliamperage  per  se.  In  some  cases  they  have 
gone  so  far  as  to  take  tubes  adapted  to  operation  with  say  40  ma.  and  run  them  at  1 0  ma. 

There  is  no  harm  in  doing  this;  but  it  is  easy  to  show  by  actual  experiment  that, 
provided  the  same  voltage  and  the  same  exposure  time,  reckoned  in  milliampere 
seconds,  are  used,  radiographs  made  with  a  40-ma.  tube  at  10  ma.  cannot  be  distin- 
guished from  those  made  at  40  ma. 

It  is  true  that,  unless  the  operator  is  equipped  with  a  time  switch  adapted  to  the 
measurement  of  short  time  intervals,  he  will  time  his  exposures  more  accurately  when 
using  low  milliamperage  and  longer  intervals. 

The  main  advantage,  however,  to  be  derived  from  the  use  of  low  milliamperage 
comes  from  the  fact  that  it  makes  possible  the  use  of  a  tube  with  a  small  focal  spot. 
It  is  easy  to  demonstrate  that,  with  satisfactory  immobilization  of  the  subject,  and  the 
same  focal-spot-plate  distance,  the  same  voltage  and  the  same  exposure  in  milliampere- 
seconds,  the  definition  is  always  better  the  smaller  the  focal  spot. 

130 


SHIELD  FOR  THE  RADIATOR  TUBE 

The  main  advantage  of  the  split  glass  shield  which  has  been  developed  for  the 
radiator  tube,  consists  in  the  fact  that  it  completely  surrounds  the  tube  and  is  made 
of  thick  glass  having  a  high  lead  content. 

In  case  the  focal  spot  happens  to  be  exactly  in  the  plane  of  separation  of  the  two 
halves  of  this  shield,  there  is  seen  to  be  a  very  thin  beam  of  roentgen  rays  escaping 
through  the  joint.  This  leakage  is  so  slight,  however,  that  it  would  seen  to  be  unim- 
portant. It  can,  moreover,  be  completely  stopped  by  seeing  that  the  focal  spot  is  not 
in  the  plane  of  separation  One  of  the  advantages  of  the  present  design  lies  in  the  fact 
that  it  makes  possible  the  condition  that  each  half  shield  shall  fit  together  with  every 
other  half  shield  to  make  a  pair. 

The  method  of  supporting  the  tube  in  the  shield  has  come  in  for  deserved  criticism. 
An  experimental  model  has  been  made  in  which  the  outer  ends  of  the  glass  are  threaded 
to  take  hard-rubber  screw  caps.  These  caps  force  tapered  split  bushings  of  some 
resilient  material,  such  as  cork,  in  between  the  arms  of  the  X-ray  tube  and  the  shield. 
This  holds  the  tube  securely  in  position  in  the  shield  and  prevents  rotation. 

EFFECT  OF  THE  RADIATOR  TUBE  ON  THE  DESIGN  OF  GENERATING  APPARATUS 

While  it  is  early  to  make  many  predictions  concerning  the  new  generating 
apparatus  which  will  be  developed  for  operating  this  tube,  the  following  statements 
seem  to  the  writer  to  be  conservative. 

Much  fluoroscopic  work  and  much  radiographic  work  will  be  done  with  a  siimple 
transformer  outfit  operated  from  a  lamp  socket.  Such  small  outfits  will  be  very  sumple 
and  can  be  made  almost  fool-proof. 

There  is  a  splendid  field  of  usefulness  for  a  small  light-weight  hand-portable  out  fit 
operating  on  this  system,  to  use  in  the  home  of  the  patient  who  cannot  be  moved  to  a 
roentgen  ray  laboratory.  Preliminary  experiments  seem  to  indicate  that  a  suitable 
small  transformer  weighing  not  more  than  40  or  50  pounds  can  be  made  for  this  service. 

In  the  field  of  dental  radiography,  it  seems  certain  that  the  outfits  of  this  type  which 
have  already  been  developed  by  different  manufacturers  will  find  a  wide  field  of  usefulness . 

CONSTRUCTION  OF  THE  TUBE 

% 

The  forced  large-scale  production  of  the  tube,  incident  to  its  war  use  and  certain 
war-time  conditions,  brought  about  several  radical  changes  in  tube  construction. 

The  fact  that  an  insufficient  number  of  skilled  glass-blowers  was  available  made  it 
necessary  to  develop  special  machines  with  which  most  of  the  glass-working  could  be 
done  by  girl  operators.  Help  was  also  obtained  by  getting  from  the  glass  works  mold- 
blown  parts,  such  as  the  cathode  side-arm,  and  the  anode-  and  cathode-support  tubes, 
instead  of  making  these  by  hand  from  glass  tubing. 

About  twelve  dollars  worth  of  platinum  was  needed  in  every  anode  and  at  one  time 
the  radiator  tube  enjoyed  the  unenviable  reputation  of  being  the  biggest  war-time  con- 
sumer of  platinum  that  there  was.  Research  work  which  had  been  carried  on  for  several 
years  finally  resulted  in  the  production  of  an  entirely  satisfactory  substitute  for  platinum 
in  this  field,  consisting  of  an  alloy  of  iron  and  nickel  covered  with  copper. 

The  early  exhaust  work  called  for  one  skilled  operator  for  each  tube  on  the  pumps, 
to  control  the  current  supplied  to  the  tube.  Experimental  work  in  this  field,  however, 
finally  resulted  in  the  development  of  a  very  simple  exhaust  system  which  automati- 
cally accomplishes  what  the  best  operator  had  done.  The  system  consists  of  a  small 
high-tension  transformer  connected  to  the  tube  terminals.  In  series  with  the  primary  of 

131 


this  transformer  is  a  large  ballast  resistance,  the  function  of  which  is  to  lower  the 
amount  of  energy  supplied  to  the  tube  as  gas  is  liberated  from  the  glass  and  the  elec- 
trodes. The  filament-current  transformer  is  operated  in  parallel  with  the  primary  of 
the  high-tension  transformer  and,  as  a  result  of  this  connection,  and  the  presence  of 
the  ballast  resistance,  the  filament  temperature  is  automatically  lowered  whenever 
gas  is  liberated  during  the  exhaust.  The  entire  operation  is  carried  out  without 
adjustment  of  either  the  ballast  resistance  or  the  filament  current  control. 


132 


APPARATUS  FOR  PORTABLE  RADIOGRAPHY* 
By  W.  D.  COOLIDGE 

INTRODUCTION 

For  cases  which  cannot  safely  or  conveniently  be  moved  to  an  X-ray  laboratory, 
the  need  has  long  been  felt,  by  the  medical  profession,  of  X-ray  apparatus  suitable  to 
conveniently  transport  to,  and  operate  in,  the  home  of  the  patient. 

In  hospital  work,  also,  there  is  a  need  for  portable  X-ray  apparatus  which  can  be 
conveniently  taken  to,  and  operated  at,  the  bedside  in  any  private  room  or  ward. 

The  extent  to  which  such  apparatus  will  be  used  in  the  future  will  depend  upon 
the  state  of  development  of  the  apparatus  at  the  time  in  question.  It  will  depend  upon 
such  factors  as  the  electrical  and  X-ray  efficiency,  ease  of  portability,  convenience  in 
handling,  controllability  and  reliability. 

The  problem  is  not  new,  but  its  importance  seems  to  justify  a  great  deal  of  careful 
and  extended  consideration  and  research.  Although  the  work  of  the  author  and  his 
associates,  in  this  field,  is  not  yet  completed  and  is  in  many  ways  imperfect,  the 
following  general  discussion  of  the  problem  and  the  description  of  an  experimental 
portable  apparatus,  together  with  the  considerations  which  have  led  to  its  present 
form,  may  be  helpful  to  the  advancement  of  the  art. 

GENERAL  CONSIDERATIONS 

The  presence  or  absence  of  an  existing  electric  lighting  or  power  circuit  has,  of 
course,  nothing  to  do  with  the  desirability  of  having  X-ray  diagnosis,  but  it  has  much 
to  do  with  the  form  of  apparatus  required.  For  this  reason,  this  section  will  be  treated 
under  two  headings : 

(a)  Where  there  is  no  Existing  Electrical  Supply  Circuit.     In  this  case,  recourse 
may,  at  the  present  time,  be  had  to  the  "U.  S.  Army  Portable  Outfit."  l 

For  civilian  practice,  it  is  clearly  desirable  to  have  something  of  easier  portability 
than  this,  and  for  this  reason,  apparatus  similar  to  that  described  below  could  well  be 
used  to  replace  the  instrument  box,  tube,  and  tube  holding  mechanism  of  the  "U.  S. 
Army  Portable  Outfit." 

(b)  Where  an  Existing  Electric  Supply  Circuit  is  Available.     This  is  the  condition 
which  has  been  predicated  throughout  the  following  discussion. 

1.  Weight  and  Size  Limitations.  It  seems  desirable  that  there  shall  be  no  single 
piece  of  the  apparatus  which  cannot  be  conveniently  carried  by  one  man.  Experience 
shows  that  a  package  weighing  43  pounds  and  having  a  thickness  of  7  inches  and  a 
depth  of  15  inches  and  a  length  of  14  inches  can,  when  provided  with  a  suitable  handle, 
be  conveniently  carried  for  short  distances  (say  a  couple  of  hundred  feet)  and  up  and 
down  stairs.  With  a  given  weight,  an  increase  in  thickness  is  always  troublesome  and, 
for  going  up  and  down  stairs,  length  and  depth  also  deserve  serious  consideration,  as 
they  determine  whether  the  arm  of  the  person  carrying  the  package  can  or  cannot  be 
fully  extended.  It  seems  undesirable  to  have  any  single  package  appreciably  exceed 
45  pounds  in  weight. 

*  From  The  Journal  of  Roentgenology,  II,  pp.  149-176  (1919). 

1  W.  D.  Coolidge  and  C.  N.  Moore,  General  Electric  Review,  Vol.  21,  pp.  60-67  (1918). 

133 


As  the  outfit  must  be  readily  transportable  in  an  automobile,  it  seems  desirable 
to  limit  the  length  of  the  longest  package  to  the  width  of  the  tonneau  of  a  small  car, 
that  is,  to  about  36  inches. 

2.  Capability  of  Stand-ing  Mechanical  Abuse.     As  a  portable  outfit  is  necessarily 
subjected  to  a  great  deal  of  mechanical  vibration  during  use  and  transportation,  it 
should  be  as  rugged  in  construction  as  is  possibly  consistent  with  the  required  light 
weight.  Provision  must,  of  course,  be  made  in  suitable  carrying  cases  for  any  necessarily 
fragile  parts. 

3.  Water-proof  ness.     As  the  outfit  must,  obviously,  often  be  taken  out  in  the 
rain,  it  is  desirable  that  every  part  shall  either  be  rainproof  or  provided  with  some 
waterproof  cover. 

4.  Amount  of  Electrical  Energy  Available.     The  operating  electric  current  will, 
in  general,  be  taken  from  a  lighting  circuit  and  the  connection  will  be  made  to  the 
nearest  lamp-socket  or  base-board  receptacle.      This  will  usually  mean  a  110-volt 
circuit  fused  for  either  6  or  10  amperes.  The  total  amount  of  energy  available  will  then 
be  about  660  watts  (the  amount  used  in  the  electric  flat-iron).     For  radiographic 
exposures  of  one  second  or  less,  about  twice  this  amount  of  energy  can  be  used  without 
blowing  6  ampere  fuses.     Longer  exposures  with  this  larger  amount  of  energy  can, 
of  course,  be  made  by  replacing  the  usual  6  or  10  ampere  house  fuses  with  some  of 
larger  capacity. 

5.  High  Efficiency  Desirable.     As  the  amount  of  electrical  energy  available  is  so 
definitely  limited,  the  questions  of  electrical  and  X-ray  efficiency  are  clearly  matters  of 
prime  importance. 

6.  Best  Voltage  for  X-ray  Tube.     As,  with  a  given  amount  of  energy  supplied  to 
the  tube,  the  photographic  action  of  the  X-rays  produced  increases  directly  with  the 
voltage,  it  is  clearly  desirable  to  use  as  high  a  voltage  on  the  tube  as  is  consistent 
with  the  production  of  satisfactory  radiographs.    This  appears  to  be  about  60,000  volts. 

7.  Size  of  Focal  Spot  in  X-ray  Tube.     For  best  definition  in  radiography,  the 
size  of  the  focal  spot  in  the  tube  should  always  be  as  small  as  can  be  used  with  the 
amount  of  energy  to  be  employed.    As,  for  portable  work,  not  more  than  10  milli- 
amperes  at  60,000  volts  is  available,  this  will  mean  a  focal  spot  diameter  of  about 
3/8  inch  (3  mm.). 

8.  Rigid  Tube-support.     It  will  be  possible  to  get,  in  portable  work,   the  fine 
definition  which  comes  from  the  use  of  such  a  small  focal  spot.    To  realize  on  this 
possible  gain,  however,  it  will  be  necessary  to  take  adequate  precautions  to  guard 
against  too  much  mechanical  vibration  of  the  tube.    (An  amplitude  of  vibration  of  as 
little  as  Y£  inch  would,  for  example,  reduce  the  sharpness  of  definition  from  that 
obtainable  with  a  10-milliampere  tube  to  that  of  a  30-milliampere  tube.    This  calls 
then,  for  a  very  rigid  tube-support. 

9.  Low  Tension  Winding  of  Transformer  or  Coil  should  be  Insulated  from  Ground. 
This  is  made  desirable  by  the  fact  that,  in  most  cases,  one  side  of  the  lighting  circuit  is 
already  connected  to  the  earth.     If  such  a  circuit  were  connected  to  a  transformer 
primary,  one  of  whose  terminals  was  already  connected  to  earth,  it  would  cause  a 
short-circuit  on  the  line  unless  it  happened  that  the  grounded  side  of  the  line  was 
connected  to  the  grounded  side  of  the  transformer.    The  necessity  of  finding  out,  by 
suitable  test,  which  side  of  the  line  is  grounded  is  obviated  if  the  transformer  primary 
is  itself  not  connected  to  earth. 

10.  Sufficient  Range  of  Electrical  Control  to  Adapt  Outfit  to  Operation  on  All  Lighting 
Circuits.     The  voltage  of  lighting  circuits  varies  somewhat  in  "different  cities,  and  in 

134 


different  parts  of  the  same  city  and  is  also  different  in  a  given  place  at  different  times. 
Furthermore,  the  voltage  which  must  be  supplied  to  the  primary  of  a  transformer 
or  coil  for  a  given  secondary  voltage  is  not  independent  of  the  load,  but  increases  with 
it.  It  is  desirable  that  one  should  always  be  able  to  supply  to  the  tube  terminals  the 
same  high  tension  voltage  regardless  of  what  the  line  voltage  at  that  particular  place 
happens  to  be  at  that  particular  time  and  regardless  of  the  milliamperage  which  he 
chooses  to  use.  For  this  reason,  there  should  be  a  control  apparatus,  and  its  range 
should  be  adequate. 

11.  Quietness  of  Both  Mechanical  and  Electrical  Operation.     This  seems  especially 
desirable  in  portable  work;  for  those  patients  who  cannot  safely  be  taken  to  an  X-ray 
laboratory  are  in  general  in  poor  condition  to  stand  noises  caused  either  by  the  mechan- 
ism of  an  X-ray  outfit  or  by  high-tension  discharges.    This  condition  is  further  inten- 
sified by  the  fact  that,  owing  to  the  small  size  of  the  average  bedroom,  the  whole 
outfit  is  very  close  to  the  patient. 

12.  Sufficient    Mechanical    Flexibility    in    the    Tube-holding     Mechanism.      The 
tube-holding  mechanism  should  have  ample  flexibility  so  that  the  tube  can  quickly 
and  readily  be  brought  to  any  desired  position,  in  order  that  the  patient  may  be 
disturbed  as  little  as  possible. 

13.  Ship-shape  Condition  of  High  Tension  Circuit.     The  condition  and  surround- 
ings attending  portable  work  are  certain  to  be  very  varied.  To  safeguard  both  patient 
and  operator  from  accidental  electrical  shock,  it  is,  therefore,  even  more  than  usually 
desirable  that  all  parts  of  the  high  tension  circuit  should  be  in  as  ship-shape  a  condition 
as  possible. 

14.  Current  Limiting  Device.     To  further  safeguard  patient  and  operator  from 
the  high  tension,  some  form  of  overload  circuit-breaking  device  should  be  used  in  the 
low -tension  circuit.    In  case  a  hurran  body  were  accidentally  interposed  between  the 
high  tension  conductors,  the  house  fuses  might  eventually  blow,  but  a  good  mechanical 
circuit-breaker  would  operate  much  more  rapidly  and  hence  give  better  protection. 

The  overload  circuit-breaker  also  affords  a  good  deal  of  electrical  protection  to  the 
X-ray  apparatus  itself. 

15.  X-ray  Protection.     In  the  past,   portable  work  has  often  been  done  with 
entirely  inadequate  protection  for  the  operator,  and  this  appears  to  be  one  of  the 
reasons  why  so  little  portable  work  has  been  done.    It  is  clearly  desirable  to  have  as 
good  X-ray  protection  in  the  home  of  the  patient  or  in  the  hospital  as  in  the  X-ray 
laboratory. 

16.  Stereoscopic  Tube-shift.     In  many  portable  cases  it  will  be  difficult  to  move 
the  patient  so  as  to  get  the  customary  right  angle  projection.    Provision,  therefore, 
should  be  made  for  stereo-radiography. 

17.  Connections  for  Low  Tension  Circuit.     The  terminals  of  the  connecting  cables 
should  be  so  designed  that  the  apparatus  cannot  be  connected  up  wrong.    The  ter- 
minals should  also  be  sufficiently  rugged  so  that  they  can  be  stepped  on  without  being 
injured. 

IS.  Precautions  Necessary  to  Guard  Against  Tube  Breakage.  Experience  teaches 
that  even  in  the  X-ray  laboratory  many  more  tubes  are  broken  in  handling  than  in  use. 
In  portable  work,  the  amount  of  handling  which  the  tube  will  receive  will  be  much 
greater  than  in  laboratory  work,  and  as  a  result,  breakage  will  be  greater  unless  careful 
consideration  is  given  to  the  design  of  the  tube,  the  tube-holder  and  the  tube-carrying 
case. 

135 


DESCRIPTION  OF  AN  EXPERIMENTAL  PORTABLE  OUTFIT 

The  experimental  apparatus  described  below  is  the  result  of  an  attempt  to  comply 
with  the  requirements  laid  down  in  the  preceding  section. 

1.  General  Description.     The  complete  alternating  current  outfit,  as  it  stands, 
ready  to  be  taken  out,  is  shown  in  Fig.  1,  and  again  set  up  ready  for  use,  in  Fig.  2. 
In  Fig.  2,  the  lamp  socket  which  serves  as  the  source  of  current  is  at  the  right  and 
above  the  picture.  A  cable  is  seen  leading  from  the  lamp  socket  down  to  the  instrument 
box  on  the  chair.   From  the  left  side  of  the  instrument  or  control  box,  another  cable  is 
seen  leading  to  the  transformer  which  stands  on  the  base  of  the  wooden  tube  stand. 
From  the  transformer,  insulated  conductors  (a  single  one  at  the  anode  end  and  a  double 
one  at  the  cathode  end)  carry  the  high  tension  current  to  a  new  self -rectifying  radiator 
type  of  X-ray  tube.   On  the  top  of  the  instrument  box  the  single  control  handle  is  seen 
at  the  front  left-hand  corner  and  the  push-button  X-ray  switch  at  the  front  right-hand 
corner. 

2.  The  Tube.     The  regular  type  of  hot-cathode  tube,  together  with  the  special 
lead-glass  shield  which  goes  with  it,  weighs  about  7  pounds.    Experiments  with  this 
combination  showed  that,  because  of  its  heavy  weight,  a  relatively  heavy  and  bulky 
tube-stand  was  required  for  a  sufficiently  rigid  support.    For  this  reason,  the  develop- 
ment of  the  special  small  tube,  shown  in  Fig.  3,  was  undertaken.    This  tube  has  an 
overall  length  of  14  inches.  The  bulb  has  an  inside  diameter  of  2  inches  and  an  outside 
diameter  of  2]/%  inches.  Except  for  the  transparent  window,  the  whole  tube  is  made  of 
glass  having  a  very  high  lead  content  (55  per  cent  by  weight  of  element  lead,  equal  in 
protective  power  to  one-fourth  its  thickness  of  sheet  lead).    The  bulb  is  ^  inch  thick 
and  therefore  offers  the  same  X-ray  protection  as  y£  inch  of  sheet  lead. 

This  tube  then  requires  no  external  X-ray  shield  and  offers  the  same  protection  as 
does  the  regular  3%  inch  radiator  tube  when  the  latter  is  equipped  with  its  special 
heavy  shield. 

It  had  been  the  original  plan  to  use  a  small  tube  made  of  the  ordinary  thin-walled 
lime  glass  and  to  surround  this  with  a  close-fitting  thick-walled  lead-glass  shield.  The 
experiment  was  tried,  however,  of  making  the  bulb  of  the  X-ray  tube  of  the  thick- 
walled  lead-glass  intended  for  the  shield,  putting  in  a  thin-walled  lead-free  window 
for  the  passage  of  the  desired  bundle  of  rays. 

There  was  a  great  deal  of  difficulty  encountered  in  making  the  first  experiment. 
The  lead-glass  used  was  a  quarter  of  an  inch  thick  and  was,  for  this  reason,  very  hard 
for  the  glass  blower  to  handle.  Unless  heated  very  slowly  and  uniformly,  it  cracked. 
It  was,  furthermore,  because  of  its  high  lead  content,  unlike  ordinary  glass,  in  that  as 
it  was  heated  it  suddenly  became  very  soft  instead  of  having  a  long  temperature  range 
through  which  it  was  plastic.  To  make  matters  still  worse,  the  available  lead-free  glass 
for  windows  has  a  considerably  higher  melting  point.  Such  tubes  were  finally  success- 
fully produced,  however.  Upon  operating  them,  it  was  found  that,  quite  contrary  to 
expectations,  they  were  much  better  than  tubes  of  the  same  size  made  of  thin  lime 
glass.  They  ran  with  less  crackling  noise  than  any  of  the  many  tubes  with  which  they 
were  compared.  It  at  first  seemed  possible  that  this  might  be  due  to  these  first  thick- 
walled  tubes  having,  for  some  unknown  reason,  a  better  vacuum  than  the  tubes  with 
which  they  were  compared.  Later  experiments,  however,  showed  that  the  better 
result  could  not  well  be  ascribed  to  this  cause.  It  was  also  not  due  to  the  substitution 
of  lead-glass  for  lime-glass,  for  tubes  made  of  thin-walled  lead-glass  were  no  better 
than  those  made  of  the  customary  thin-walled  lime-glass.  It  was  then  due  to  the  use  of 
thick-walled  glass,  and  this  conclusion  was  experimentally  confirmed  by  making  some 

136 


tubes  from  lime-glass  bulbs  having  a  quarter-inch  wall  thickness.    They  behaved  as 
well  as  the  thick-walled  lead-glass  bulbs. 

The  effect  of  the  bulb  thickness  on  the  quietness  of  operation  of  a  tube  is  probably 
to  be  ascribed  either  to  the  leakage  of  electricity  through  the  glass  or-to  the  electrostatic 
condenser  action  of  the  negatively  charged  inner  surface  of  the  bulb  and  the  slightly 
conducting  outer  surface.  Further  experiments  will  be  undertaken  to  clear  up  this 
point. 

The  cathode  of  the  new  tube  is  the  same  as  that  of  the  regular  10-milliampere 
radiator  tube.  The  anode  is  also  the  same  except  that  it  is  shorter.  The  path  which 
the  heat  has  to  travel  from  the  focal  spot  to  the  radiator  is  5}/£  inches  in  the  new,  and 
8^4  inches  in  the  earlier  tube.  For  continuous  operation,  then,  the  capacity  of  the 
smaller  tube  should  be  somewhat  greater  than  that  of  the  larger  one. 

The  window  has  essentially  the  same  X-ray  transparency  as  does  the  wall  of  the 
bulb  in  the  earlier  types  of  tube. 

The  new  tube,  complete  with  radiator,  weighs  2  pounds,  that  is,  less  than  one-third 
as  much  as  the  earlier  type  fitted  with  a  shield  giving  the  same  amount  of  X-ray 
protection. 

3.  The  Transformer.  The  transformer  is  shown  completely  assembled  with 
cover  in  Fig.  1  and  uncovered  in  Fig.  2.  Inside  views  are  shown  in  Figs.  4  and  5. 

It  is  an  oil  insulated  transformer  and  delivers  10  milliamperes  at  60,000  volts 
(useful).  Complete  with  wooden  base  and  carrying  cover,  it  weighs  43  pounds.  It  is 
especially  designed  with  reference  to  the  operation  of  a  self-rectifying  tube,  or,  in  other 
words,  to  have  for  a  given  weight,  a  minimum  inverse  voltage. 

When  the  development  work  on  this  transformer  was  started,  the  lightest  oil- 
insulated  transformer  available  for  this  service  weighed,  without  any  protecting  cover 
or  base,  72  pounds. 

Of  the  various  steps  taken  to  reduce  weight,  the  most  effective  consisted  in  so 
shaping  the  transformer  case  as  to  make  it  conform,  as  closely  as  possible,  to  the  high 
tension  coils.  This  very  considerably  reduced  the  required  amount  of  oil. 

With  this  reduction  in  the  volume  of  the  transformer,  the  safe  bringing  out  of 
the  low  tension  leads,  in  such  a  manner  that  they  cannot  get  too  close  to  the  high 
tension  parts  of  the  system,  becomes  an  important  problem.  This  was  solved  in  the 
following  manner : 

A  U-shaped  metal  piece  (25  in  Fig.  6),  was  introduced  into  each  end  of  the  trans- 
former case.  The  tongue,  26,  of  this  U-shaped  piece  fits  tightly  in  the  space  between 
the  end  of  the  core  and  the  end  wall  of  the  transformer  case  and  holds  the  U-shaped 
member  in  the  position  shown  in  Fig.  4,  which  is  an  elevation  of  the  transformer 
(with  cover  removed).  This  is  also  shown  in  the  plan  view  given  in  Fig.  5.  The  U- 
shaped  pieces,  together  with  the  side  walls  of  the  case,  thus  form  rectangular  metal 
enclosures,  completely  walled  off  from  the  other,  or  high-tension,  compartment. 
The  primary  leads,  28,  are  brought  up  through  the  right-hand  compartment,  which 
they  enter  through  an  opening,  27,  in  one  of  the  lower  corners.  The  other  two  low 
tension  leads,  29,  are  connected  to  the  inner  ends  of  the  two  high  tension  coils  and 
lead  out  through  the  left-hand  compartment  to  a  milliammeter  in  the  instrument  box. 
The  diagram  of  transformer  connections  is  shown  in  Fig.  7,  in  which  28  is  the  low  ten- 
sion coil  and  13,  13  are  the  two  high  tension  coils. 

Besides  serving  to  keep  the  low  tension  leads  away  from  the  high  tension  part  of  the 
transformer,  the  U-shaped  members,  25,  serve  two  other  important  functions.  Extend- 
ing as  they  do  from  the  core  to  the  cover,  they  help  to  hold  the  core  down.  Placed  in 

137 


position,  they  electrostatically  shield  the  high  tension  coils  from  the  sharp  edges  of  the 
case,  21,  22,  23  and  24,  for  the  outer  surfaces  of  the  inner  legs  of  these  members  lie  flush 
with  the  surfaces  17  and  18  of  the  transformer  case.  They,  therefore,  reduce  the  danger 
of  electrical  breakdown  at  these  points. 

The  low-tension  coil  is  wound  on  a  treated  paper  tube  and  is  made  integral  with 
this  by  subsequent  treatment  with  a  special  black  varnish  which  is  baked  until  it  is 
hard.  This  tube  slips  tightly  over  the  core.  The  high  tension  coils  are  likewise  impreg- 
nated with  black  varnish  and  baked.  As  a  result  of  this  treatment,  it  becomes  impos- 
sible for  these  coils  to  suffer  from  mechanical  vibration.  To  keep  them  from  moving 
laterally  on  the  primary,  the  treated  paper  support-tubes  on  which  they  are  wound, 
and  with  which  they  are  made  integral  by  the  black- varnish  treatment,  are  left  long 
enough  to  completely  fill  the  core  window.  Rotation  of  the  secondaries  is  also  prevented 
*by  the  support  tubes.  They  are  so  shaped  at  their  outer  ends  that  the  core  prevents 
their  rotation. 

Current  for  heating  the  cathode  spiral  is  derived  from  a  separate  coil  wound  over 
one  otf  the  high  tension  secondaries. 

As  a  safety  spark  gap,  to  protect  the  transformer  from  high-voltage  surges,  a 
sphere  gap,  set  to  spark  over  at  90,000  volts,  is  used  for  the  transformer  terminals. 

The  sphere-gap  has  been  used  in  preference  to  a  point-gap  for  the  following  reasons : 

(1)  A  sphere-gap  is  quicker  in  .action  than  a  point-gap  and  its  use,  therefore, 
imposes  less  electrical  strain  on  the  insulation  of  the  transformer. 

(2)  It  avoids  the  corona  attendant  on  the  use  of  pointed  electrodes. 

(3)  It  saves  space  (the  electrodes  of  a  point -gap  would  have  to  be  about  9  inches 
apart,  while  the  corresponding  spacing  of  the  2-inch  spheres  is  3}/2  inches). 

To  prevent  warping  of  the  cover  of  the  transformer,  which  would  cause  a  change 
in  the  spacing  of  the  terminals  of  the  safety-gap,  the  cover  is  made  of  bakelite. 

The  removable  cover,  to  which  the  carrying  handle  is  attached,  is  made  to  extend 
below  the  bakelite  cover,  to  shed  rain.  In  use,  it  is  clamped  in  place  by  four  wing-nuts 
which  screw  on  pivoted  bolts  attached  to  the  upper  corners  of  the  transformer  case. 

The  base  of  the  transformer  is  made  of  wood,  to  better  withstand  mechanical 
abuse.  Jt  is  made  large  (7X14  inches)  for  stability. 

4.  The  Control  of  Tube  Voltage  and  Current.  Auto-transformer  control  is  used. 
The  auto-transformer,  together  with  a  special  auto-transformer  switch,  is  located  in 
the  instrument  box  (see  Fig.  8).  By  means  of  this  combination,  operated  from  the 
handle  at  the  left  side  of  the  box,  it  is  possible  to  always  deliver  a  definite  voltage,  say 
100  volts,  to  the  transformer  primary  even  though  the  line  voltage  may  be  as  much  as 
15  per  cent  above  or  below  this  value. 

The  voltmeter,  shown  at  the  left  rear  corner  of  the  instrument  box  indicates  the 
voltage  delivered  by  the  auto-transformer. 

The  rnilliammeter  is  connected  in  to  the  middle  point  of  the  secondary  of  the 
X-ray  transformer. 

Nothing  further  would  be  needed  for  the  control  of  voltage  and  milliamperage, 
were  it  not  for  the  fact  that  the  cathodes  in  different  tubes  are  not  exactly  alike  and 
that  the  voltage  of  the  filament  heating  circuit  will  not  be  exactly  the  same  for  all 
transformers.  To  take  care  of  this  complication,  a  little  rheostat,  diagramatically 
shown  at  9  in  Fig.  7,  is  built  into  the  reel  used  on  the  cathode  side.  As  this  is  in  the 
high  tension  circuit,  it  is  desirable  for  the  safety  of  the  operator,  that  it  should  be  used 
as  seldom  as  possible.  For  this  reason  and  also  to  prevent  the  setting,  once  made,  from 
being  accidentally  disturbed,  a  perforated  metal  cover  is  provided  which  is  kept  locked 

138 


securely  in  place  over  the  rheostat  by  means  of  a  thumb  screw.  See  Fig.  9,  which 
shows  one  face  of  the  cathode  reel  with  rheostat  uncovered. 

The  X-ray  transformer  is  calibrated1  by  means  of  a  suitable  sphere-gap  and 
kenotron.  The  calibration  of  a  certain  experimental  transformer  was,  for  example, 
as  follows : 

For  5  m.a.  at  60,000  volts  (effective),  use  100  volts  on  the  primary. 

In  using  a  tube  for  the  first  time  on  this  transformer,  the  rheostat  in  the  filament 
circuit  would  be  adjusted  until  with  a  voltmeter  reading  of  100,  the  tube  is  carrying 
5  milliamperes.  The  cover  is  then  fastened  over  the  rheostat  and  it  should  never  be 
necessary  to  remove  it  as  long  as  the  same  tube  and  transformer  are  used. 

In  subsequent  work,  and  regardless  of  what  the  line  voltage  may  be,  the  desired 
milliamperage  is  secured  by  merely  setting  the  auto-transformer  switch.  With  this 
method  of  operation,  it  will  always  be  found  that  the  primary  voltage  is  near  enough 
to  the  figure  given  in  the  calibration.  (If  the  milliamperage  is  correct  and  there  is  an 
error  of  5  per  cent,  for  example,  in  voltage,  the  error  in  exposure  will  amount  to  only 
10  per  cent,  an  amount  which  in  ordinary  work  will  be  negligible.) 

This  simple  method  of  control  is  made  possible  by  the  fact  that  the  electron 
emission  from  the  hot-cathode  changes  so  very  rapidly  with  temperature  and,  hence,  in 
this  case,  with  the  voltage  applied  to  the  primary  of  the  X-ray  transformer.  This  is 
illustrated  by  the  following  table  which  shows  how  the  milliamperage  changed,  in  an 
actual  experiment,  as  the  applied  voltage  was  changed  by  means  of  the  auto-trans- 
former switch.  The  rheostat  in  the  filament  circuit  was  not  touched  during  the  experi- 
ment. The  third  cclumn  gives  the  values  of  the  high  tension  voltage  (effective). 

PORTABLE  TRANSFORMER  No.  11 


Primary 
Volts" 

Tube  Current 
(Milliamperes) 

Tube  Voltage 
(Useful) 

100 
102 

2.8 
5.0 

60,000 
61,000 

104 

6.0 

61,500 

107 
110 

7.4 
8.5 

62,000 
63,000 

111 

9.5 

64,300 

112 

10.0 

65,000 

An  increase  of  12  per  cent  in  primary  voltage  has  here  caused  an  increase  of  260 
per  cent  in  milliamperage  and  an  increase  of  only  8.3  per  cent  in  secondary  voltage. 

o.  Circuit  Breaker.  This  is  shown  in  Fig.  10.  It  is  placed  inside  of  the  instrument 
box  and  is  connected  in  series  with  the  main  low- voltage  circuit.  When  tripped,  it 
can  be  reset  by  means  of  the  vertical  rod  which  leads  up  through  thf  cover  and  is 
surmounted  by  a  large  button. 

This  circuit  breaker  is  a  very  important  part  of  a  portable  or  any  other  X-ray 
outfit.  It  reduces  the  danger  to  patient  and  operator  from  accidental  electric  shock. 
Furthermore,  even  with  the  best  hot  cathode  tubes  so  far  produced,  it  will  occasionally 
happen  that  there  is  a  high  voltage  surge  which  causes  a  flame  discharge  to  take  place 
across  the  safety  gap  on  the  transformer.  This  amounts  to  a  practical  short  circuit 
on  the  line  and,  in  the  absence  of  a  circuit  breaker,  the  current  drawn  from  the  lamp 
socket  would  suddenly  jump  from,  say,  6  amperes  to  perhaps  50.  As  a  result,  some 
fuse  or  fuses  in  the  garret  or  the  basement,  as  the  case  may  be,  would  burn  out.  The 

1  See  General  Electric  Review,    XXI,    page  58  (1918). 

139 


circuit  breaker,  which  operates  at  about  20  amperes  will,  with  an  overload,  act  quickly 
enough  to  save  a  6  ampere  fuse.  Its  use  will  save  the  operator  much  time  which  \vould 
otherwise  be  spent  in  locating  house  fuses. 

6.  Switches.     The  main  circuit  is  closed  by  means  of  a  push-button  switch  located 
in  the  right  front  corner  of  the  instrument  box  and  opened  later  by  a  motor-driven 
adjustable  relay  (time-switch)  which  is  seen  at  the  front  of  the  instrument  box.    The 
push-button  switch  is  a  three  leaved  one,  with  a  6-ohm  resistance  between  the  middle 
and  lowest  leaves.    Upon  pressing  it,  the  upper  and  middle  leaves  first  make  contact, 
thus  closing  the  low  tension  circuit  through  the  resistance  which  is  then  short-circuited 
as  the  middle  and  lowest  leads  come  together.    Unless  the  circuit  is  closed  through 
resistance,  in  this  way,  there  will  be  a  very  high  voltage  surge  whenever  it  happens  that 
the  circuit  is  closed  at  or  near  the  peak  of  the  voltage  wave. 

7.  The  Tube-stand.     Before  taking  up  the  development  of  the  stand  described 
below,  a  good  deal  of  consideration  was  given  to  the  method  which  has  been  most 
frequently  used  in  the  portable  work  of  the  past.    This  method  consists  in  using  an 
adjustable  arm  carrying  the  tube-holder  at  one  end  and  attached  at  the  other  to  the 
case  of  the  coil  or  other  high  potential  source.    The  method  necessitates  placing  the 
coil  on  the  edge  of  a  chair  or  table.    This  gives  a  rather  precarious  support.    Further- 
more, the  chair  or  table  presumably  has  four  legs.    If  the  joints  are  good  and  strong, 
this  is  bad,  as  the  support  will  then  rock  until  a  wedge  is  put  under  the  leg  which  is 
off  of  the  floor.   If  the  table  or  chair  is  sufficiently  weak  in  the  joints  it  will,  under  the 
weight  of  the  coil,  stand  on  four  legs,  but  such  a  weak  piece  of  furniture  does  not  give 
a  good  firm  support  either.    Besides  the  question  of  the  bearing  on  the  floor,  there  is 
also  the  question  of  having  the  coil  properly  supported  on  the  chair  which  may  have 
a  seat  of  almost  any  shape.   Besides  the  above  objections,  the  chair  or  table  support  is 
clumsy  to  move  around. 

The  stand  which  was  finally  developed  is  made  of  wood.  It  is  shown  in  Fig.  2  and 
again,  disassembled,  in  Fig.  11.  Wood  was  chosen  rather  than  metal  because  of  its 
being  an  electrical  insulator  and  because  it  stands  mechanical  abuse  better  than  a 
thin-walled  metal  structure  of  equal  weight. 

The  triangular  base  has  a  3-point  support  and  is  made  as  large  as  can  be  accom- 
modated in  the  special  suitcase  designed  to  carry  the  photographic  material  and  some 
of  the  detachable  parts  of  the  outfit. 

The  upright  post  has  a  metal  casting  attached  to  the  bottom  and  this  is  securely 
clamped  to  the  base  by  means  of  two  machine-screws  with  wing-shaped  heads.  The 
considerations  which  determine  the  position  of  the  upright  on  the  base  are  the  follow- 
ing: 

(1)  In  position  for  use  at  the  bedside,  the  wooden  upright  should  come  between 
the  metal  of  the  bed  and  the  transformer  terminals.   This  definitely  places  the  upright 
essentially  in  the  middle  of  the  base. 

(2)  For  maximum  stability,  the  upright  should  be  so  located  on  the  base  as  to 
bring  the  transformer,  as  it  is  in  Fig.  2,  over  two  of  the  base  supports. 

As  there  is  no  brace  extending  from  the  upright  to  the  front  point  of  the  base,  the 
latter  can  be  pushed  forward,  under  even  the  lowest  bed,  until  the  upright  almost 
touches  the  side  of  the  bed.  In  this  way  the  overhang  required  of  the  extension  arm  is 
reduced  to  a  minimum. 

The  upright  is  made  of  large  section  for  torsional  rigidity. 

The  extension  arm  is  secured  by  two  clamp-screws.  It  pivots  about  and  can  be  slid 
through  the  one  in  the  upright.  The  second  clamp-screw,  with  trie  short  arm  to  which 

140 


it  is  attached,  makes  it  unnecessary  to  set  the  first  clamp-screw  up  very  tight  and, 
furthermore,  it  sets  a  safe  limit  to  the  downward  travel  of  the  tube. 

Motion  of  the  tube  about  three  different  axes,  at  right  angles  to  one  another,  is 
secured  through  three  clamp-screws.  In  addition  to  this,  the  tube  may  be  easily 
rotated  about  its  own  length  axis. 

For  the  stereoscopic  shift,  a  small  clamp-screw  which  attaches  the  U-shaped 
tube-holder  to  the  balance  of  the  tube-holding  mechanism  is  loosened  and  the  tube 
may  then  be  easily  moved  on  the  arc  of  a  circle,  of  24-inch  radius,  whose  center  is  the 
center  of  the  photographic  plate  when  the  latter  is  in  position.  To  prevent  possible 
movement  of  the  whole  stand,  during  the  operation  of  making  the  tube-shift  and 
changing  plates,  a  little  fiber  cup,  attached  to  the  base,  is  brought,  by  rotation  of  the 
steel  member  which  carries  it,  under  the  front  caster. 

As  a  mechanical  safeguard  for  the  tube  and  to  render  manipulation  easier,  a  helical 
spring  is  used  at  the  point  where  the  tube-carrying  mechanism  is  attached  to  the  end 
of  the  extension  arm.  This  spring  counteracts  the  effect  of  gravity  in  tending  to  cause 
rotation  about  the  clamp  screw  at  this  point.  Without  this  spring,  the  tube-carrying 
mechanism  can,  when  the  thumb-screw  in  question  is  loosened,  swing  down,  under 
the  influence  of  gravity,  so  far  that  the  tube  strikes  against  either  the  extension  arm 
or  the  upright  post. 

Heavily  insulated  cable  is  used  for  the  high  tension  leads.  This  is  to  prevent 
corona  with  its  attendant  noise  and  odor.  It  also  helps  greatly,  by  its  weight,  in 
reducing  tube-vibration,  cutting  it  down  to  less  than  half  of  what  it  would  otherwise 
be. 

The  use  of  large  cable  necessitates  large  special  reels.  To  keep  the  high  tension 
leads  away  from  the  bed  and  the  patient,  these  reels  are  placed  high  upon  the  stand. 
They  are  supported  at  the  ends  of  a  wooden  cross-arm  and  are  widely  spaced  to  allow 
much  freedom  of  movement  of  the  tube  without  having  the  leads  come  too  close 
together.  The  cable  used  on  the  cathode  side  is  a  two  conductor  cable  and  two  separate 
circuits  have  to  be  provided  through  the  reel  on  this  side.  One  circuit  leads  through 
the  spring  of  the  reel  to  the  fixed  axis  and  the  other  leads  through  a  metal  brush,  and 
a  slip-ring  shown  in  Fig.  2,  on  the  inner  face  of  the  reel. 

The  cross-arm  and  tube-holder  must  be  kept  dry,  as  they  have  to  support  the  full 
electrical  potential  of  the  high  tension  circuit.  They  are  carried  in  the  suitcase.  For 
use  in  places  where  the  humidity  is  very  high,  it  may  prove  desirable  to  make  them  of 
bakelite. 

The  type  of  caster  used  for  the  base  is  very  important.  The  casters  must  work 
nicely  on  polished  floors  and  on  thick  rugs.  Of  many  different  types  which  have  been 
tried,  the  "A.  B.  C.  Ball"  casters  have  proved  to  be  by  far  the  best.  Equipped  with 
these,  the  tube-stand  may  be  readily  moved  by  taking  hold  of  it  anywhere.  With 
any  other  type  of  caster  which  has  been  tried,  it  would  be  necessary  to  use  a  larger 
base  as,  otherwise,  the  stand  could  be  too  easily  overturned  in  attempting  to  push  it 
about.  Contrary  to  what  might  have  been  predicted,  the  easy  mobility  which  these 
casters  give,  also  greatly  reduces  the  trouble  from  tube  vibration.  This  may  readily 
be  seen  by  striking  the  side  of  the  tube-holder  with  the  hand.  When  this  is  done,  it 
will  be  found  that,  with  the  other  casters  which  have  been  tried,  the  upright  post  is 
given  a  torsional  deflection  and  then  continues  to  execute  a  series  of  torsional  vibra- 
tions. While  these  may  be  of  small  angular  magnitude,  they  produce  a  considerable 
linear  movement  of  the  tube  at  the  end  of  its  long  support -arm.  When  the  experiment 
is  tried  with  "A.  B.  C."  casters,  the  result  is  entirely  different.  The  mobility  of  the 

141 


system  is  then  so  great  that,  under  the  force  of  a  sidewise  blow,  the  whole  stand  moves, 
thus  eliminating  the  torsional  vibration  of  the  upright  post. 

The  greater  mobility  of  the  "A.  B.  C.  Ball"  caster  appears  to  be  due  partly  to  the 
fact  that  it  is  a  ball-bearing  device,  but  mainly  to  the  fact  that  it  permits  free  move- 
ment in  any  direction  without  first  having  to  be  forced  into  a  certain  definite  position 
with  respect  to  this  direction. 

8.  Special  Hospital  Base  for  Tube-stand.     For  hospital  work,   a  different   base, 
mounted  on  three  light  and  relatively  large  rubber-tired  wheels,  is  used  for  the  stand. 
A  fourth  wheel,  smaller  than  the  others,  is  mounted  a  short  distance  back  of  the  main 
rear  wheels.   It  is  so  placed  that  it  is  normally  about  a  half  inch  above  the  floor.   With 
the  transformer  in  position,  the  center  of  gravity  of  the  stand  is  only  slightly  in  front 
of  the  axis  of  the  main  rear  wheels.    By  a  slight  downward  pressure,  exerted  on  the 
rear  end  of  the  extension  arm,  the  front  wheel  may  be  lifted  from  the  floor.     The 
stand  is  then  on  two  wheels  and  can  readily  be  turned  sharply  to  either  side.    The 
small  rear  wheel  merely  serves  to  keep  the  stand  from  being  tipped  over  backwards. 
In  this  way  the  axes  of  all  four  wheels  can  be  rigidly  mounted  instead  of  having  to  be 
pivoted  and  steered. 

9.  A  Special  Alternative  Tube-holder.     A  special  tube-holder  has  been  developed 
which  must  be  tested  out  in  competition  with  the  simpler  one  shown  in  Fig.  11.    It  is 
made  of  bakelite  and  is  shown  in  Fig.  12.    The  X-ray  tube  is  flexibly  mounted  inside 
of  the  holder,  by  means  of  the  two  disks  of  sponge  rubber  seen  in  the  figure  on  the  side 
arms  of  the  tube.    It  may  be  accurately  centered  in  the  holder  and  then  fixed  in  place 
by  a  single  clamp-nut,  attached  to  a  bakelite  strip  which  is  carried  by  a  thin  bakelite 
ring  which  surrounds  the  sponge  rubber  disk  used  on  the  cathode  side-arm.     The 
bakelite  tube  of  the  holder  is  rotatable  in  two  brass  rings. 

The  total  weight  of  the  holder  is  1^  pounds.  The  advantages  which  it  offers  are 
the  following : 

(1)  It  serves  as  a  carrying  case  for  the  tube  which  can  be  put  as  it  is,  without 

special  packing  precautions,  in  the  suit-case. 

(2)  It  can  be  used  for  special    work  where  it  is   desirable  to  intercept  the  light 

coming  from  the  tube. 

(3)  It  serves  for  the  ready  attachment  of  filters  and  diaphragms  and,  if  desired,  of 

a  pointer  for  indicating  the  direction  of  the  central  beam  of  X-rays. 

The  disadvantage,  if  it  has  one,  is  that  it  conceals  the  glass  X-ray  tube  and  may 
hence  lead  to  more  mechanical  abuse  than  the  latter  would  otherwise  get. 
As  shown,  it  is  not  adapted  to  stereo-work,  but  it  can  readily  be  made  so. 

10.  Operation  on  Direct  Current  Circuits.     For  direct  current  work,  a  converter 
(rotary)  has  to  be  used.  This  is  shown  in  its  carrying  case  in  Fig.  13.   Unless  a  starting 
box,  or  its  equivalent,  is  employed,  the  starting  of  this  rotary  will  blow  6-ampere  fuses. 
To  obviate  the  need  of  using  a  regular  starting  box,  which  is  needlessly  large  and 
heavy,  a  two  point  switch  and  a  single  small  resistance  unit  are  employed.     Upon 
turning  this  switch  to  the  first  point,  the  converter  is  connected  to  the  line  through 
the  resistance  unit  and  on  the  second  point  the  resistance  is  cut  out. 

The  connections  are  so  arranged  that  the  use  of  the  converter  does  not  in  any  way 
affect  the  calibration  of  the  balance  of  the  electrical  part  of  the  outfit. 

The  converter  weighs  about  35  pounds. 

The  direct  current  taken  from  the  line,  for  60,000  volts  (useful)  at  the  tube,  will  be 
about  6  amperes  for  5  milliamperes  and  10  amperes  for  10  milliamperes  of  high  tension 
current. 

142 


1 1 .  The  Suit-case.     This  is  divided  up  into  compartments  and,  with  its  contents, 
will  weigh  about  40  pounds. 

12.  Technique.     This  will  be  the  same  as  that  of  any  other  tube  operating  with 
the  same  current  and  voltage. 

In  this  connection,  attention  should  be  called  to  the  fact  that  the  capabilities  of 
such  an  outfit  may  be  tremendously  increased  by  the  use  of  screens  and  especially  so 
by  the  use  of  the  Duplitized  film  and  double  screens.  With  this  last  combination, 
for  example,  with  10  milliamperes,  chest  radiographs  may  be  made  at  a  distance  of 
28  inches,  in  one-half  second;  mastoids  at  a  distance  of  20  inches,  in  two  and  one-half 
seconds;  and  frontal  sinuses  at  a  distance  of  18J/2  inches  in  nine  seconds. 

With  double  screens  having  a  speed  factor  of  say  6,  a  10-milliampere  tube  is  made 
to  do  work  which,  for  the  same  speed,  would  require  without  screens  a.  GO-milliampere 
tube.  The  result  is  the  vastly  improved  definition  which  goes  with  the  10-milliampere 
focal  spot,  which  is  less  than  half  as  large  as  that  required  for  the  60-milliampere  tube. 
Furthermore,  the  radiograph  made  with  the  double  screens  will,  if  the  same  voltage  is 
employed  in  both  cases,  show  markedly  greater  contrast. 

In  closing,  the  author  wishes  to  acknowledge  his  indebtedness  to  the  many  members 
of  the  laboratory  staff  and  to  the  many  factory  engineers  who  have  contributed  to  the 
development  work  described  in  the  paper. 

It  is  also  a  pleasure  to  thank  Mr.  C.  N.  Moore  for  his  helpful  efforts  in  testing  the 
outfit  in  the  laboratory,  in  the  home  of  the  patient  and  in  the  hospital. 

He  also  wishes  to  express  his  appreciation  of  the  many  courtesies  received  from 
Dr.  Charles  G.  McMullen,  whose  sympathetic  co-operation  has  been  of  the  greatest 
assistance. 

Research  Laboratory,  General  Electric  Company,  Schenectady,  X.  Y. 


143 


144 


Fig.  2 


14o 


Fig.  3 


Fig.  4 


-22 


O        O        O       O        O        O 


/7 


146 


Fig.  6 


MILU-AMMETCR 


-34 


vwvwv   wwwv 


Fig.  7 


Fig.  8 


147 


Fig.  9 


148 


Fig.  10 


1-19 


Fig.   11 


Fig    12 


150 


Fig.  13 


151 


ROENTGEN-RAY  SPECTRA* 

BY  ALBERT  W.  HULL 

The  following  is  a  preliminary  report  of  measurements  of  roentgen  ray  spectra.  It 
is  presented  as  a  first  contribution  to  exact  roentgenology  and  an  example  of  what  can 
be  done  in  the  way  of  accurate  measurement  of  the  quality  and  intensity  of  roentgen 
rays  produced  by  a  standard  tube 

Our  ability  to  make  these  exact  measurements  of  roentgen  ray  quality  is  due  to  five 
great  discoveries  of  the  last  three  years. 

The  first,  by  Professor  Laue  and  his  colleagues,  is  the  proof  that  roentgen  rays 
are  the  same  in  nature  as  ordinary  light;  that  is,  electric  waves  of  definite  wave-length. 
In  light,  the  quality, — that  is,  color,  absorbability,  actinic  and  chemical  effect — depends 
only  upon  wave-length,  and  recent  experiments  have  shown  the  same  to  be  true  of 


roentgen  rays.  This  gives  us 
a  standard  of  quality — the 
wave-length. 

The  second  great  discovery 
is  a  tube  that  behaves  the 
same  yesterday,  to-day  and 
forever,  and  every  tube  exactly 
like  every  other  one — by  Dr. 
Coolidge. 

Third,  the  discovery  of  a 
source  of  constant  high  volt- 
age of  unlimited  power,  by 
means  of  the  kenotron — by 
Dr.  Langmuir  and  his  associ- 
ates. These  last  two,  the 
standard  tube  and  standard 
power  to  run  it,  give  us  some- 
thing definite  to  measure. 

Fourth,  a  roentgen  ray 
spectrometer  which  enables 
us  to  break  up  a  beam  of 


00000000000  00000  0000 
- — 100.000  Yo/tsA  C  — - 


-JM/WVWW—  (V)  - 


\—/ooo0o  Vo/ts  ac  —  -I 


Fig.  1 


roentgen  rays  into  its  con- 
stituent wave-lengths,  in  the 
same  way  as  a  beam  of  light  is 
broken  up  into  its  constituent 
colors  by  a  prism — by  Pro- 
fessor Bragg  and  his  son. 

Fifth,  the  proof  by  Pro- 
fessor Barkla,  of  London,  and 
his  associates,  that  a  reliable 
measure  of  the  intensity  of  a 
beam  of  roentgen  rays  is  the 
ionization  it  produces  in  a 
chamber  large  enough  to  com- 
pletely absorb  it. 

These  last  two,  the  spec- 
trometer and  ionization 
chamber,  give  us  the  means 
of  resolving  the  roentgen  ray 
beam  into  its  constituentwave- 
lengths  and  measuring  the  in- 
tensity of  each  wave-length. 


Regarding  the  first  and  last  of  these  discoveries,  namely,  the  proof  that  roentgen 
rays  are  identical  with  light  except  in  wave-lengths,  and  that  a  large  ionization  chamber 
can  measure  intensity,  I  need  only  say  that  the  evidence  is  so  good  that  it  is  accepted  by 
all  roentgen  ray  physicists. 

The  reproducibility  of  the  hot  cathode  tube  can  be  verified  directly  by  any  roent- 
genologist,  but  it  is  worth  mentioning  that  any  lack  of  reproducibility  is  a  theoretical 
impossibility  in  a  well  exhausted  hot  cathode  tube.  At  the  same  voltage  and  current  it 
must  give  the  same  rays,  both  in  quality  and  quantity,  at  all  times. 

The  other  two  "implements,"  the  high  voltage  apparatus  and  the  spectrometer, 
will  be  described  in  detail. 


*  Copyright,  1915,  by  American  Journal  of  Roentgenology. 


153 


Fig.  2.     92,000  Volts,  55  Milliamperes.     Lower  Curve  Primary  A.  C.  Wave,  Upper  Curve  D.  C.  Voltage 

THE  HIGH  VOLTAGE  APPARATUS 

The  arrangement  of  apparatus  for  obtaining  high  constant  voltage  with  large 
power  is  shown  in  Fig.  1. 

Alternating  current  of  2,000  cycles  and  150  volts  flows  through  the  operating  switch 
5  to  transformer  T,  which  boosts  the  voltage  to  100,000  volts.  This  100,000  volt 
current  flows  through  the  kenotrons  KI  and  A'2,  which  rectify  it,  through  the 
choking  coils  LI  and  L2,  to  the  roentgen  ray  tube  X.  The  small  condensers  C\ 

and  Ci  (capacitv   •      :  microfarad  each),  together  with  the  choking  coils  LI  and  L» 
1000 

(inductance  about  200  henries  each)  smooth  out  the  voltage  oscillations  that  re- 
main after  rectifying,  so  that  the  voltage  at  the  terminals  of  the  roentgen  ray  tube  is 
constant  within  less  than  1%  (for  5  kw.  or  less)  and  this  constant  voltage  is  measured 
by  an  ordinary  voltmeter  V  of  very  high  resistance. 

The  constancy  of  the  voltage  is  shown  in  the  oscillograms*  of  Figs.  2  and  3,  which 
were  taken  with  a  water  resistance  in  parallel  with  the  tube.  The  lower  curve  in  each 
is  the  2,000  cycle  primary  voltage,  the  upper  curve  the  voltage  at  the  terminals  of 


Fig.  3.     50,000  Volts,  65  Milliamperes.     Lower  Curve  Primary  A.  C.  Wave,  Upper  Curve  D.  C.  Voltage 


*  It  was  found  necessary  to  retouch  these  oscillograms  in  order  to  get  plates  for  publication. 

154 


the  tube.  In  Fig.  2  the  voltage  is  92,000  and  the  current  55  milliamperes,  making  a  load 
of  5  kw.  Under  these  conditions  it  can  be  seen  that  the  voltage  fluctuates  about  1%. 
The  fluctuations  are  much  small  for  lighter  loads,  and  can  be  still  further  reduced  at 
voltages  below  50,000  by  connecting  the  two  kenotrons  in  parallel  and  using  the  middle 
point  of  the  transformer.  Fig.  3  shows  the  result.  Here  the  voltage  is  50,000  and  the 
current  65  milliamperes,  making  3.2  kw.  The  base  line  for  the  constant  voltage  has 
been  made  coincident  with  that  of  the  primary  2,000  cycle  wave.  The  fluctuations 
probably  do  not  show  in  the  reproduction.  They  are  about  1/10  of  1%. 

These  tests  prove  that  we  have  a  source  of  voltage  sufficiently  constant  for  our  pur- 
pose. In  fact,  it  is  as  constant  as  that  of  any  low  voltage  direct  current  generator,  and 
is  read  with  the  same  precision  on  the  same  kind  of  voltmeter. 


Earbh 


Earth 


Fig.  4  (a) 


Fig.  4  (b) 


THE  ROENTGEN  RAY  SPECTROMETER 

The  principle  of  the  spectrometer  can  best  be  understood  by  comparison  with  the  or- 
dinary light  spectrometer.  If  one  wishes  to  analyze  a  beam  of  visible  light  into  its  con- 
stituent colors  one  lets  it  fall  on  a  prism,  as  in  Fig.  4  (a).  By  placing  the  eye  at  R.,  Y 
or  V  one  will  then  see  red,  yellow,  or  green,  respectively,  provided  these  colors  are  pres- 
ent in  the  beam,  and  one  can  estimate  by  the  eye  the  relative  intensity  of  the  different 
colors.  Thus,  if  the  beam  of  light  came  from  an  incandescent  carbon  filament  the 
yellow  would  be  most  intense;  if  from  a  mercury  arc  the  green  would  predominate. 

For  accurate  work,  such  as  is  done  in  our  lamp  factories,  the  eye  is  not  a  sufficiently 
reliable  judge  either  of  color  or  intensity.  It  is  therefore  replaced  by  a  telescope.  The 
color  of  each  constituent  of  the  light  is  recorded  in  terms  of  its  wave-length,  which 
can  be  calculated  from  the  angle  A  of  the  telescope,  and  the  intensity  is  measured  by 
a  photometer. 


155 


\ 


Spectra, 
Mazda  i 


Fig.  5 


A  record  is  thus  obtained  of  the  intensity  of  each  wave-length  (that  is,  each  color, 
wave-length  being  a  more  precise  definition  of  color)  in  the  beam  of  light.  The  results 
are  generally  shown  graphically,  by  plotting  the  wave-lengths  as  abscissas  (horizontal) 
and  the  intensities  as  ordinates  (vertically).  Fig.  5  shows  the  spectrum,  as  it  is  called, 
of  a  tungsten  Mazda  lamp  measured  and  plotted  in  this  way.  The  shaded  portion  is  the 
visible  part,  that  to  the  right  being  the  so-called  infra  red;  that  is,  wave  lengths  too  long 
to  affect  the  eye. 

The  roentgen  ray  spectrometer  operates  on  exactly  the  same  principle.  Since  the 
wave-lengths  are  much  shorter  than  those  of  visible  light,  the  glass  prism  has  to  be 

replaced  by  a  crystal  of  rock  salt  or 

iceland  spar,  and  the  photometer  by 
something  that  is  sensitive  to  short 
wave-lengths,  such  as  a  photographic 
plate  or  an  ionization  chamber.  We 
use  the  later  because  it  is  the  more 
accurate. 

The  spectrometer  is  shown  in  Fig. 
4  (b).  The  roetgen  ray  tube  is  com- 
pletely enclosed  in  a  lead  box  ^  inch 
thick.  The  rays  emerge  from  the  box 
through  the  small  hole  0,  pass  through 
narrow  slits  5,  and  52,  and  fall  on  the 
crystal  C.  This  crystal  reflects  each  wave-length  in  a  different  direction.  The  reflected 
rays  enter  the  ionization  chamber  /,  where  they  make  the  gas  electrically  conducting 
and  cause  a  current  to  flow  from  the  outside  of  the  chamber,  which  is  highly  charged, 
to  the  central  electrode  E,  and  thence  through  the  sensitive  ammeter  A  to  earth.  This 
current  is  proportional  to  the  intensity  of  the  rays  entering  the  chamber.  The  angle  6 
at  which  the  chamber  is  set  tells  which  wave-length  is  entering  the  chamber.  Thus, 
by  moving  the  chamber  around 
gradually  from  small  to  large  angles, 
and  recording  the  current  for  each 
angle,  we  have  a  record  of  the  in- 
tensity of  each  wave-length  in  the 
spectrum  which  we  are  examining. 

The  Roentgen  Ray  Spectra. — Fig.  6 
shows  the  spectrum  thus  obtained  of 
a  tungsten  target  at  40,000  volts  and 
1  milliampere,  plotted  in  the  same 
way  as  that  of  the  hot  tungsten  fila- 
ment, Fig.  5.  The  scale  of  wave- 
length, which  is  laid  off  horizontally,  is  measured  in  the  same  units  as  for  the  visible 
spectrum,  the  so-called  Angstrom  unit,  which  is  a  hundred-millionth  of  a  centimeter. 
Thus  the  wave-lengths  given  by  a  roentgen  ray  tube  can  be  compared  directly  with 
those  of  an  incandescent  lamp.  The  intensity  of  each  wave-length  is  equal  to  the 
vertical  distance  from  the  corresponding  point  on  the  horizonal  axis  up  to  the  curve. 
For  example,  the  intensity  of  the  rays  of  length  .3  is  zero;  that  is,  no  rays  of  this  length 
are  produced  by  the  tube  at  40,000  volts.  The  rays  of  length  .35  have  an  intensity  55, 
those  of  length  .42  have  the  greatest  intensity  of  all,  namely,  97.  The  units  in  which 
intensity  is  measured  are  chosen  arbitrarily,  since  only  relative  intensity  is  desired  here. 
They  will  later  be  referred  to  some  standard. 


\ 


Fig.  6 


156 


\ 


Fig.  7  shows  the  spectra  obtained  at  five  different  voltages,  from  20,000  to  40,000. 
Two  things  are  very  striking.  As  voltage  increases,  the  average  wave-length  decreases 
very  rapidly,  which  means  increased  penetrating  power  a  fact  already  well  known. 
And  at  the  higher  voltages  the  intensity  is  greater  for  all  wave  lengths. 

Thus  an  increase  of  voltage  increases  not  only  the  penetrating  radiation  but  the  soft 
radiation  also,  and  it  is  only  by  the  use  of  some  selective  filter,  such  as  aluminium, 
which  absorbs  the  long  wave-lengths  more  than  the  short  ones,  that  one  can  hope  to  ob- 
tain penetrating  radiation  without  soft  radiation.  Even  so,  the  amount  of  aluminium 
necessary  is  very  large,  much  larger  than  has  commonly  been  supposed.  Fig.  8  shows 
the  spectrum  at  70,000  volts,  first 
without  any  filter  (curve  1)  and  then 
with  3  mm.  of  aluminium  (curve  2). 
It  can  be  seen  that  the  aluminium  has 
reduced  the  intensity  of  the  soft  radia- 
tion (long  wave-lengths)  much  more 
than  that  of  the  penetrating,  but 
there  is  still  an  enormous  amount  of 
soft  radiation  left.  This  could,  of 
course,  be  much  further  reduced  by 
the  use  of  still  more  aluminium,  but 
not  without  cutting  down  the  in- 
tensity of  the  useful  penetrating  rays 

to  a  small  fraction  of  its  original  value.  With  the  new  powerful  water-cooled  tube 
described  elsewhere  in  this  issue  it  will  be  possible  to  use  this  method  and  stilt  have  left 
sufficient  intensity  for  practical  work.  This  will  open  up  a  new  field  in  deep  therapy 
and  roentgenography.  In  addition,  it  is  hoped  by  the  use  of  particular  target  materials 
which  give  a  very  powerful  characteristic  radiation  for  some  particular  wave-length, 

to  obtain  still  purer ' '  monochromatic 
rays    of    high   penetration.      Experi- 
ments in  this  line  will  be  repoited  in 
the  near  future. 


Fig.  7 


*^ 


IK  ">,„•', 


Fig.  s 


COMPARISON  OF  ALTERNATING  AND 
DIRECT  VOLTAGE  SPECTRA 

The  work  described  above  was  all 
done  with  constant  potential.  It  is 
important  to  know  whether  the  same 
results  can  be  obtained  with  an  alter- 
nating potential  (rectified,  either  by 

the  tube  itself  or  by  some  reliable  rectifier)  ,  and  whether  the  spectra  obtained  by  the 
two  methods  (constant  and  fluctuating  potential  respectively)  are  sufficiently  alike  so 
that  work  done  with  one  can  be  compared  with  work  done  with  the  other.  In  answering 
"Yes"  to  this  question,  I  want  to  be  very  careful  to  be  clearly  understood.  There  are 
a  great  many  kinds  of  alternating  potential,  which  are  very  different  from  each  other. 
Fig.  9  shows  two  typical  voltage  curves.  Curve  1  is  the  voltage  given  by  an  induction 
coil  with  mercury  interrupter;  Curve  2  that  of  a  standard  25-kw.  transformer. 

It  is  probable  that  the  spectra  given  by  these  two  machines  at  the  same  max- 
imum voltage  and  same  current  would  not  be  the  same,  or  even  approximately  so.  I 
think,  though  as  yet  I  have  tried  it  only  for  the  sine  wave  and  constant  potential,  that  I 


157 


could  find  a  voltage  and  a  current  for  the  one,  at  which  it  would  give  a  spectrum  nearly 
the  same  as  that  given  by  the  other  at  a  different  (definite)  voltage  and  current.  For  ex- 
ample, the  one  machine  might  require  70,000  maximum  volts  and  2  milliamperes  to 
give  the  same  spectrum  as  the  other  machine  at  60,000  volts  and  1  milliampere.  Know- 
ing this  conversion  factor,  it  would  be  possible  to  reproduce  with  machine  (2)  work  done 
on  machine  (1). 

But  I  wish  to  emphasize  that  the  conversion  factor  is  likely  to  be  big,  for  machines 
differing  greatly  in  wave  form,  and  can  by  no  means  be  neglected.  In  order  to  use  the 
conversion  factor  it  is  of  course  necessary  to  be  able  to  measure  the  true  maximum 
voltage,  a  question  fully  discussed  by  Dr.  Coolidge  elsewhere  in  this  issue. 

The  simplest  form  of  alternating  potential,  and  at  the  same  time  the  most  repro- 
ducible and  most  easily  measured,  is  the  simple  sine  wave  (curve  2,  Fig.  9).  This  is  the 


Curve  1.      Oscillogram  of  Induction  Coil  Voltage 


Fig.  9 
Curve  2.     Voltage  Wave  of  Standard  Transformer 


kind  of  voltage  wave  that  is  always  obtained  from  a  good  transformer  when  not  over- 
loaded. If,  in  addition,  we  use  as  rectifier  a  kenotron,  which  simply  suppresses  the 
lower  half  of  the  wave  without  changing  the  upper  (useful)  half,  or,  in  case  of  the  hot 
cathode  tube,  we  make  the  tube  act  as  its  own  rectifier,  then  we  have  a  fluctuating 
potential  of  definite  form  for  which  the  "conversion  factor"  can  be  obtained.  This 
has  been  done  for  the  voltage  of  70,000,  and  Fig.  10  shows  the  results.  The  voltage 
factor  is  unity;  that  is,  the  maximum  A.  C.  voltage  is  the  same  as  the  D.  C.  voltage.* 
The  current  factor  is  %  ;  that  is,  3  milliamperes  A.  C.  at  70,000  volts  maximum  are  re- 
quired to  give  the  same  spectrum  as  2  milliamperes  D.  C.  at  70,000  volts.  Further 
data  regarding  the  factor  of  conversion  from  sine  wave  to  constant  potential  will  be 
published  soon,  in  connection  with  the  constant  potential  spectra. 

In  conclusion,  I  wish  to  emphasize  the  fact  that  it  is  the  wave-length  of  the  rays  used, 
and  not  voltage  or  spark  gap,  that  determines  penetration,  therapeutic  action,  and  all 
other  effects.  In  order  to  duplicate  work  it  is  necessary  to  use  a  beam  containing  the  same 
wave-lengths  in  the  same  relative  intensity,  that  is,  the  same  spectrum.  With  constant 
potential,  or  fluctuating  potential  of  definite  wave-form,  it  is  probab.e  that  the  un- 
filtered  spectrum  depends  only  upon  voltage.  If  this  proves  to  be  so,  the  only  measure- 
ments which  an  operator  will  have  to  make  under  these  conditions  are  voltage,  thickness 


*  The  exact  equality  of  the  voltages  cannot  be  affirmed  without  further  experiment,  a»  the  transformer  used  was  not 
fitted  with  a  special  voltage  coil.      The  difference,  if  any,  is  certainly  very  small. 

158 


of  filter,  and  milliampere-minutes.  This  question  is  being  carefully  investigated,  and 
will  be  reported  on  in  the  near  future.  The  one  thing  that  can  be  stated  with  absolute 
certainty  at  present,  and  which  is  of  fundamental  importance,  is  that  if  one  works 
always  with  rays  having  the  same  spectrum,  the  only  variable  is  milliampere-minutes. 

It  will  probably  never  be  possible  to  compare  completely  by  any  simple  numerical 
ratio  the  effects  of  roentgen  ray  beams  that  have  different  spectra.  For  they  differ  not 
in  magnitude  but  in  kind.  It  is,  therefore,  desirable  to  devise  means  for  obtaining 
always  the  same  spectrum,  or  one  of  two  or  three  standard  spectra,  and  doing  all  work 
under  one  of  these  standard  conditions.  It  is  to  be  hoped  that  the  recently  appointed 
committee  on  standardization  will  take  up  at  an  early  date  the  problem  of  standardiz- 
ing roentgen  technique,  so  that  all  work  done  by  members  of  this  Society  will  be  done 
with  one  of  two,  or  at  most  three,  spectra — spectra,  not  voltages. 


-1 
--*-V 


Fig.  10 


159 


THE  HIGH  FREQUENCY  SPECTRUM  OF  TUNGSTEN* 

BY  ALBERT  W.  HULL  AND  MARION  RICE 

Moseley1  has  shown  that  the  high  frequency  spectrum  given  out  by  the  target  of  an 
X-ray  tube  consists  of  two  series  of  lines  superimposed  upon  a  continuous  spectrum. 
The  lines,  which  are  known  as  the  K  and  L  series,  respectively,  are  characteristic  of  the 
material  of  the  target.  Moseley  measured  the  wave  lengths  of  most  of  the  A'  lines  for 
elements  having  atomic  weights  between  aluminum  and  silver,  and  of  the  L  lines  for 
elements  from  calcium  to  gold,  and  showed  that  for  all  the  lines  measured  the  square 
roots  of  the  frequencies  of  corresponding  lines  are  proportional  to  the  atomic  numbers 
of  the  elements  emitting  them.  Malmer2  has  added  to  this  list  the  K  lines  of  six  more 
elements  between  silver  and  lanthanum,  and  W.  H.  Bragg3  and  others  have  studied  in 
great  detail  the  lines  of  a  few  of  these  elements,  especially  rhodium  and  platinum. 

The  continuous  or  band  spectrum  was  observed  qualitatively  by  Moseley  (I.e.),  and 
its  short  wave-length  limit  at  different  voltages  measured  by  Duane  and  Hunt4  who 
found  this  limiting  frequency,  vmax,  to  be  exactly  proportional  to  the  voltage  on  the 
tube,  and  given  accurately  by  the  quantum  relation  hvmaK  =  eV,  where  V  is  the  voltage 
on  the  tube,  e  the  charge  of  an  electron,  and  h  Planck's  constant. 

The  spectrum  of  tungsten  is  of  special  interest  on  account  of  its  use  as  target 
material  in  X-ray  tubes,  and  it  has  been  the  subject  of  several  recent  investigations. 
Barnes5  measured  the  L  lines,  but  was  unable  to  find  any  K  lines,  although  his  voltage, 
90,000,  was  sufficiently  high  for  their  excitation.  Gorton6  also  measured  the  L  lines. 
Rutherford,  Barnes  and  Richardson,7  using  the  coefficient  of  absorption  method, 
measured  the  effective  wave-length  of  the  "end  radiation,"  i.e.,  the  short  wave-length 
limit  of  the  continuous  spectrum,  for  different  voltages  up  to  180,000,  and  found  that 
this  minimum  wave-length  did  not  decrease  continuously  with  increase  of  voltage,  but 
approached  asymptotically  a  limiting  value  of  0.172  Angstrom  units.  As  will  be  shown 
below,  the  wave-lengths  found  by  the  spectrometer  are  much  shorter,  and  do  not  ap- 
pear to  approach  any  limiting  value. 

The  spectrum  shown  in  Fig.  1  was  taken  in  the  usual  manner  with  a  rock  salt 
crystal  in  continuous  slow  rotation,  photographic  plate  stationary  at  19.13  cm.  distance 
from  the  crystal,  collimating  slits  0.2  mm.  wide  and  20  cm.  apart,  with  a  Coolidge  tube 
running  at  1  milliapmere  and  100,000  volts  constant  potential.  The  horizontal  band 

L     Designation  of  line,  D  =  Distance  from  C  in  cm.,  X  =  Wave-length  in  Angstrom 
L Xo  fti  ai  fii  e*2  Agi  03  as  Br 


D 0.964          1.332          1.496          2.614          2.932          2.994          3.332          3.948          4.386          4.494          6.524 

X 0.142         0.196         0.219         0.192         <*215         0.220         0.488         0.192         0.212         0.217         0.463 


Ag2 


D 6.886         7.368         7.604         7.662         7.884         9.074         9.224         9.382         9.558       11.10          11.19 

X 0.487         1.033         1.065         1.073         1.100         1.242         1.260         1.280         1.300         1.468          1.4SO 

gives  the  reflection  from  the  cubic  (100)  planes  of  the  crystal,  those  at  45  deg.  from  the 
dodecahedral  (110)  planes, and  those  between  from  the  tetrahexahedral  (210)  planes.etc. 

*  Copyright,  1916,  by  National  Academy  of  Sciences. 

i   Moseley,  Phil.  Mag.,  26,  210  and  1024  (1913);  27,  710  (1914). 

*  I.  Malmer,  Phil.  Mag.,  28,  787  (1914). 

'  W.  H.  Bragg,  Phil.  Mag.,  29,  407  (1915). 

«  Duane  &  Hunt,  Physic.  Rer.,  6,  166  (1915). 

5  Barnes,  Phil.  Mag.,  SO,  368  (1915). 

6  Gorton,  Physic  Rev.,  7,  203  (1916). 

7  Rutherford,  Barnes  and  Richardson,  Phil.  Mag.,  30,  339  (1915). 

161 


The  wave-lengths  of  the  lines  and  bands  in  the  horizontal  strip  are  given  in  the  ac- 
companying table.  C,  in  the  photograph,  is  the  undeviated  central  beam,  diminished  in 
intensity  by  passage  through  a  lead  strip  2  mm.  thick  and  5  mm.  wide,  placed  in  front 
of  the  plate.  The  shadow  of  this  strip  extends  only  one  fourth  of  the  distance  from  C  to 
X0  and  is  not  visible  on  the  plate. 

.  The  short  wave-length  limit  of  the  spectrum,  marked  X0,  is  the  shortest  wave- 
length that  is  produced  by  electrons  of  velocity  corresponding  to  the  operating  voltage 
(100,000)  and  its  value,  0.142  Angstrom  units,  agrees  very  closely  with  the  value  given 
by  Duane  and  Hunt's  formula  e  V  =  h vmax  =  hc/\m[n .  I  have  already  shown1  that  the 


Fig.  1 

proportionality  between  frequency  and  voltage  holds  accurately  up  to  100,000  volts, 
and  these  measurements  have  since  been  extended,  with  less  accuracy,  up  to  150,000 
volts.  The  shortest  wave-length  so  far  observed  is  SX  10~10  cm.,  or  0.08  A.U. 

The  lines  marked  oujSi,  0:2182,  and  a3^  are  the  first,  second,  and  third  order  reflections 
respectively  of  the  Ka  and  Kft  lines  of  tungsten,  the  a  line  being  a  doublet.  They  are 
more  clearly  shown  in  Fig.  2,  which  was  taken  with  narrower  slits  and  greater  dis- 
tance of  photographic  plate  from  crystal,  so  that  the  doublet  is  clearly  resolved.  In 


Fig.  2 

this  photograph  the  second  order  was  given  an  extra  exposure,  which  accounts  for  the 
apparent  band  in  the  middle.  In  the  photograph  of  Fig.  1,  all  parts  were  given  the 
same  exposure.  The  wave-lengths  of  the  lines  are  about  6%  less  than  would  be  re- 
quired by  Moseley's  formula.  Slight  deviations  from  the  formula  have  already  been 
noted  for  the  K  lines  measured  by  Moseley  and  Malmer.  All  these  lines,  including 
tungsten,  can,  however,  be  correctly  represented  by  the  empirical  formulae 

va=  1.64  X  1015^V2-10for  the  a  lines, 
and  vs  =  1.56  X  1015N2-15  for  the  ft  lines, 


1  Hull,  Physic.  Rev.,  7,  156  (1916). 


162 


where  v  is  the  frequency  and  N  the  atomic  number.    These  formulae,  while  they  have 
no  theoretical  signficance,  may  be  useful  for  interpolation. 

The  bands  which  terminate  on  the  long  wave-length  side  at  Agi  and  Ag2  are  due  to 
the  silver  in  the  photographic  plate  and  are  produced  by  the  reflection,  in  the  first  and 
second  order  respectively,  of  those  wave-lengths  which  are  capable  of  stimulating  the 
characteristic  radiation  of  silver,  that  is,  those  which  are  shorter  than  the  gamma  line 
of  silver.  The  edge  of  the  band  therefore  marks  the  position  of  the  7  line  of  silver, 


too 


90 


cuf*ve  NO-  I  = 

.  <?=  80  K  v. 
3  =  ~7O  K  v. 
60  K\/ 
E  HO  5  =  SO  K  v 


Fig.  3 

0.488  A.U.  In  the  same  way  the  band  terminated  at  Br  is  due  to  the  bromine  in  the 
photographic  plate,  and  marks  the  position  of  the  7  line  of  bromine. 

The  remaining  nine  lines,  a k,  belong  to  the  L  spectrum  of  tungsten,  and 

agree  with  those  found  by  Barnes,1  with  the  inclusion  of  two  faint  lines  not  observed  by 
Barnes.  The  line  marked  h  appears  to  be  a  doublet.  Gorton's2  values  are  all  about 
2%  smaller. 

»  Barnes.  Phil.  Mag.,  SO,  368  (1915). 
Gorton,  Physic  Rev.,  7,  203  (1916). 

163 


The  position  of  the  K  lines,  and  their  relation  to  the  general  radiation  at  different 
voltages,  has  been  studied  by  means  of  the  ionization  chamber  also.  Fig.  3  shows  the 
ionization  current  as  a  function  of  the  angle  of  incidence  of  the  rays  on  the  crystal,  for 
five  different  voltages,  and  Fig.  4  a  part  of  the  same  in  the  second  order.  The  position 
of  the  K  lines,  in  the  first,  second  and  third  order  respectively,  is  shown  by  the  dotted 
lines  marked  cti,  /3t,  as,  j32,  etc.  There  is  no  trace  of  the  lines  at  70,000  volts,  but  at 
80,000  they  are  clearly  visible,  and  increase  in  intensity  as  the  voltage  increases.  It  is 


W  TARGET 
z  '\&5  mm  W 

NaCl  CRYSTAL. 


Fig.  4 


probable  that  the  lowest  voltage  at  which  the  lines  appear  is  the  "quantum"  voltage' 
i.e.,  that  given  by  Duane  and  Hunt's  equation,  for  the  7  line,  about  70,000  volts  for 
tungsten.  This  would  be  in  harmony  with  the  mechanism  of  radiation  suggested  by 
Kossel1  and  has  already  been  found  by  Webster2  to  hold  for  rhodium.  It  may  be  men- 
tioned that  the  quantum  relations  established  by  Kossel  between  the  frequencies  of  the 
K  and  L  lines  hold  true  for  the  tungsten  lines. 

A  more  detailed  account  of  these  and  other  experiments  on  the  tungsten  spectrum, 
including  the  distribution  of  energy  in  the  continuous  spectrum,  will  be  published 
shortly  in  the  Physical  Review. 


Kossel,  Ber.  D.  Physik.  Ges.  16,  953  (1914). 
Webster,  these  Proceeding,  8,  9,0  (1916). 


164 


THE    MAXIMUM    FREQUENCY   OF   X-RAYS  AT   CONSTANT  VOLTAGES 

BETWEEN  30,000  AND  100,000.'* 

BY  ALBERT  W.   HULL 

RESEARCH  LABORATORY,  GENERAL  ELECTRIC  COMPANY,  SCHENECTADY,  N.  Y. 

In  the  Philosophical  Magazine  for  September,  1915,  Professor  Rutherford  has  re- 
ported experiments  in  which  the  maximum  frequency  of  X-rays  at  different  voltages 
was  calculated  from  the  absorption  coefficients  of  the  total  radiation  after  a  sufficient 
part  of  it  has  been  absorbed  to  make  the  absorption  coefficient  nearly  constant.  He 
found  that  the  maximum  frequencies  determined  in  this  way  did  not  increase  linearly 
with  the  voltage,  but  less  rapidly,  and  reached  a  maximum  at  140,000  volts. 


•)&* 


put 


C/rc/fs  'experimental 


Fig.  1 

The  measurements  given  below  are  taken  from  the  curves  of  energy  distribution  in 
the  X-ray  spectra  at  constant  potentials,2  which  the  author  has  been  investigating. 
They  were  taken  with  the  spectrometer,  with  an  accuracy  of  about  three  or  four  per 
cent.  Within  this  limit  of  error  the  maximum  frequencies  are  proportional  to  voltage 
and  are  given  by  the  quantum  relation3  e  V  =  hvmSL^,  where  j/max  is  the  maximum  fre- 
quency of  X-rays  produced  at  constant  potential  V,  e  is  the  charge  of  an  electron,  and  h 
Planck's  constant,  which  is  taken  as  6.59X10~17.  It  is  evident  from  the  graph  that 
there  is  no  tendency  to  fall  below  this  value  at  higher  voltages,  as  Rutherford  found. 
For  convenience  of  comparison,  Rutherford's  values  are  reproduced  on  the  dotted 
curve. 


*  Copyright,  1916,  by  American  Physical  Society. 

1  Abstract  of  a  paper  presented  at  the  Chicago  meeting  of  the  Physical  Society,  November  26,  1915. 

-  To  be  published  soon. 

3  Duane  (Phys.  Rei:,  6,  166,  Aug.,  1913)  has  already  shown  this  to  be  true  for  voltages  up  to  40,000. 


165 


Volts  X  10' 

MAXIMUM  FREQUENCY  X1018 

Theoretical 

Experimental 

24.0 

5.8 

5.78 

34.1 

8.21 

8.27 

48.8 

11.78 

12.14 

82.0 

19.8 

19.5 

89.0 

21.4 

22.4 

95.0 

22.9 

22.8 

The  cause  of  the  difference  between  Rutherford's  values  and  the  author's  is  due  to 
the  fact,  which  Duane  (/.  c.)  has  already  pointed  out,  that  the  absorption  method  does 
not  give  the  maximum  frequecy.  It  can  be  shown  from  the  energy  distribution  curves 
that  the  absorption  coefficient  method  leads  to  values  of  j/max  which  fall  below  the  true 
values  more  and  more  as  the  voltage  is  raised.  In  fact,  Rutherford's  values  can  be  cal- 
culated directly  from  the  energy  distribution  curves  and  the  law  of  absorption.  This 
question  will  be  more  fully  discussed  in  a  future  publication.  The  fact  which  it  is  de- 
sired to  point  out  here  is  that  the  maximum  frequency  for  voltages  up  to  100,000  is 
given  accurately  by  the  quantum  relation,  and  that  there  is  no  reason  to  believe  that 
it  should  not  continue  to  increase  with  voltage  indefinitely. 


166 


THE  LAW  OF  ABSORPTION  OF  X-RAYS  AT  HIGH  FREQUENCIES1* 

BY  ALBERT  W.  HULL  AND  MARION  RICE 

It  has  been  shown,  from  Barkla's  absorption  data  and  Moseley's  table  of  wave- 
lengths, that  the  coefficient  of  absorption  of  all  metals  varies  approximately  as  the  cube 
of  the  wave-length,  except  in  the  immediate  vicinity  of  one  of  the  characteristic  wave- 
lengths of  the  metal.  The  experimental  data  extends  over  a  range  of  wave-lengths  from 
4  to  0.5  A.  U.  approximately,  but  the  law  has  frequently  been  extrapolated  to  very  short 
wave-lengths  and  used  as  a  measure  of  wave-length.  It  is  important  to  know  how  far 
such  extrapolation  is  justified. 

The  measurements  given  below  were  made  on  narrow  portions  of  a  beam  of  "white 
radiation  from  a  tungsten  target,  dispersed  by  a  rock-salt  crystal  and  isolated  by  a  very 
narrow  slit  in  the  lead  face  of  the  ionization  chamber.     The  absorbing  sheets  were  15 
cm.  from  this  slit,  so  that  the  amount  of  fluorescent  and  scattered  radiation  entering  the 
chamber  was  negligible. 

The  energy  taken.from  the  beam  by  the  absorbing  sheets  consisted,  therefore,  of  two 
parts  : 

1  .  That  which  was  transformed  into  energy  of  a  different  form  or  different  wave- 
length, such  as  heat,  fluorescent  radiation,  corpuscular  radiation. 

2.  That  which  was  re-emitted  as  radiation  of  the  same  wave-length,  viz.,  the  scat- 
tered radiation.  The  observed  absorption  coefficient  fjL/p  may  therefore  be  written 

M  =  T+<T 

P     P     P 

where  r  is  what  may  be  called  the  transformation  coefficient  and  a  the  coefficient  of 
scattering. 

On  the  simple  electromagnetic  theory  we  should  expect  a  to  be  independent  of  wave- 
length and  proportional  to  the  number  of  scattering  electrons  per  unit  volume,  i.e.,  to 
the  density  approx.,  so  that  <r/  p  should  be  a  universal  constant.  This  hypothesis  is  sub- 
stantiated, within  the  limit  of  experimental  error,  by  Barkla's  measurements.  If  we 
assume  that  the  other  part  of  the  absorption,  the  "transformation  coefficient,"  varies  as 
the  cube  of  the  wave-length,  eq.  1  becomes  n/p  =  a\s+b,  where  a  is  constant  for  a  given 
absorber  between  its  absorption  bands,  and  b  is  the  same  for  all  substances  and  all  wave- 
lengths. 

Taking  a  from  Barkla's  data  on  long  wave-length  radiations,  and  6  =  0.12,  the  ob- 
served absorption  coefficeints  for  aluminum  and  copper  should  be  given  by 


( 


=  14.9X3  +  0.12, 

Al 

=  150.X3+0.12, 

Cu 

where  X  is  in  Angstroms. 

These  equations  are  plotted  in  the  figure  (full  lines),  and  agree  with  the  experimental 
values  within  experimental  error  (the  errors  were  rather  large  for  the  shortest  and  long- 
est wave-lengths  measured,  on  account  of  small  deflections).  Rutherford's  value  for 
the  absorption  in  aluminum  of,  the  shortest  7-rays  from  radium  B,  viz.,  (/z/p)Al  =  0.19 
for  X  =  0.164  A.U.,  is  also  shown  in  the  figure.  It  falls  satisfactorily  on  the  curve. 

*  Copyright,  1916.  by  American  Physical  Society. 

1  Abstract  of  a  paper  presented  at  the  Washington  meeting  of  the  Physical  Society,  April  20-21,  1916. 

167 


For  lead,  the  region  investigated  includes  one  of  the  characteristic  absorption  bands 
of  lead.  Photographs  of  the  spectrum  through  a  lead  sheet  showed  that  this  band, 
which  is  due  to  the  excitation  of  the  K  fluorescent  radiation  of  lead,  begins  at  X  =  0.149 
A.U.  For  wave-lengths  longer  than  this,  the  absorption  obeys  the  equation  (/Vp)pb 
=  430X3+0. 12  (full  curve) .  The  value  of  a  =  430  is  subject  to  some  error  on  account  of 
the  difficulty  of  measuring  the  thickness  of  the  lead  sheet.  For  wave-lengths  shorter 
than  0.149  the  data  is  not  sufficient  to  determine  the  law,  but  it  is  evident  that  ex- 
trapolation of  the  above  equation  is  not  justified. 


6J**U 


*M4 


7 


/<* 


*L 


Mass  Absorption  Coefficient  (/x/p)  for  Al,  Cu,  and  Pb,  between  X  =0.12  and  X  =0.39  Angstroms. 
Full      lines  =  theoretical  curves  for  complete  absorption  coefficient. 

Broken    "    =theoretical  curves  for  "corrected  absorption"  or  "transformation"  coefficient. 
*  =  Rutherford's  value  for  7  rays. 

Fig.  1 

If,  instead  of  dealing  with  the  coefficient  of  total  absorption,  as  above,  we  calculate 
the  "corrected  absorption  coefficient"  or  "transformation  coefficient,"  by  subtracting 
0-/p  =  0.12  from  the  observed  value  of  ju/p,  we  have  for  all  substances  and  all  wave- 
lengths thus  far  investigated  the  very  simple  law 

-  =  aX3, 

P 

where  a  is  a  constant  for  each  substance  over  the  entire  range  between  its  absorption 
bands.  This  law  is  shown  graphically  for  aluminum  and  copper  (broken  lines)  in  the  figure. 


Wave-length 


MASS  ABSORPTION  COEFFICIENT 


Angstroms 

Al.                                              Cu. 

Pb. 

Obs. 

Calc.                     Obs.                     Calc. 

Obs. 

Calc. 

0.392 

0.860 

1.02 

0.343 

0.726 

0.721 

0.294 

0.493 

0.499                3.84 

3.94 

11.1 

11.04 

0.245 

0.342 

0.339                2.24                 2.33 

6.7 

6.44 

0.221 

0.283 

0.281                1.70                 1.74 

4.63 

4.76 

0.208 

0.255 

0.254                1.39                 1.47 

3.70 

3.99 

0.196 

0.243 

0.232                1.27                 1.25 

3.40 

3.36 

0.184 

0.218 

0.213                1.07 

1.06 

2.71 

2.80 

0.172 

0.199 

0.196                0.91 

0.88 

2.32 

2.31 

0.160 

0.178 

0.181                0.79 

0.74 

1.82 

1.88 

0.147 

0.154 

0.167                0.71 

0.60 

1.50 

2.80 

0.122 

3.00 

168 


THE  X-RAY  SPECTRUM  OF  TUNGSTEN 
BY  A.  W.  HULL 

THE  NATURE  OF  X-RAYS 

Ever  since  the  discovery  of  X-rays,  in  1S96,  scholars  have  been  divided  in  opinion 
regarding  their  nature.  One  school,  lead  by  Prof.  W.  H.  Bragg,  held  that  the  rays  con- 
sisted of  high  speed  particles,  so  small  and  so  fast  that  they  could  penetrate  solid  bodies. 
Their  arguments  were  based  mainly  on  the  energy  changes  between  the  X-rays  and  the 
cathode  rays  that  produce  them.  The  other  school  considered  the  X-rays  to  be  the 
same  in  nature  as  ordinary  light,  i.  e.,  electromagnetic  waves,  and  the  principal  evidence 
in  favor  of  this  view  was  the  fact  that  X-rays  cannot  be  bent  or  deflected  by  the  strong- 
est electric  and  magnetic  fields.  The  question  has  now  been  settled  in  favor  of  the  wave 
theory,  and  it  is  a  beautiful  example  of  scientific  open-mindedness  that  Prof.  Bragg,  the 
champion  of  the  corpuscular  theory,  was  one  of  the  first  to  accept  the  decisive  evidence,-^ 
and  has  become  the  chief  exponent  of  the  wave  theory  which  he  so  long  opposedr  It  is 
interesting  to  note  that  exactly  the  same  difference  of  opinion  existed  in  Sir  Isaac  New- 
ton's time  regarding  the  nature  of  ordinary  light,  and  that  Newton,  during  his  whole 
life,  believed  in  the  corpuscular  theory.  We  may  be  sure  that  he,  too,  would  have  been 
prompt  to  change  to  the  now-accepted  wave  theory,  if  the  decisive  evidence  had  ap- 
peared in  his  life-time. 

It  is  not  the  purpose  of  the  present  article  to  describe  the  beautiful  experiments 
which  led  to  the  solution  of  this  problem— -this  has  been  ably  done  by  Prof.  Bragg  him- 
self1—  but  rather  to  present  as  vivid  a  picture  as  possible  of  the  present  theory  and  its 
interesting  consequences. 

DEFINITION  OF  SPECTRUM 

Since  light  consists  of  waves  of  electric  and  magnetic  force  traveling  through  space, 
its  quality  must  depend  on  the  lengths  of  these  waves,  and,  if  there  is  more  than  one 
wave-length,  on  their  relative  intensities.  The  spectrum  of  the  light  is  the  sum  total  of 
these  wave-lengths,  weighted  according  to  their  intensities.  The  commonest  form  of 
spectrum  is  a  photograph  of  the  beam  after  it  has  passed  through  a  prism  or  grating. 
The  prism  or  grating  separates  the  wave-lengths  and  sends  each  to  a  different  point  on 
the  photographic  plate,  where  it  produces  a  blackening  proportional  to  its  intensity. 
The  distance  measured  horizontally  along  the  spectrum  (c.f.  Fig.  3)  gives,  therefore,  the 
wave-length,  and  the  blackness*  at  that  point  the  intensity,  of  each  constituent  of  the 
beam.  If  the  intensity  of  any  particular  wave-length  is  greater  than  that  of  its  neigh- 
bors it  stands  out  as  a  black  line,  thus  producing  the  so-called  "line  spectrum"  that  is 
characteristic  of  gases  and  vapors.  Fig.  3  is  an  example.  (The  black  lines  print  as  white 
lines.)  In  the  light  from  an  incandescent  solid  body,  on  the  other  hand,  the  intensity  of 
neighboring  wave-lengths  differs  but  little,  so  that  its  spectrum  is  a  continuous  band, 

Copyright,  1916,  by  General  Electric  Review. 

1  "X-rays  and  Crystal  Structure,"  by  W.  H.  and  W.  L.  Bragg,  G.  Bell  &  Sons,  London,  1915. 
*  Fig.  3  is  made  from  a  positive  print,  so  that  blackness  is  printed  as  whiteness. 

169 


shading  off  gradually  on  each  end.  In  this  case  the  photograph  is  less  satisfactory  for 
expressing  the  intensity  relation  than  a  curve  like  Fig.  4,  which  represents  the  spectrum 
of  incandescent  tungsten  at  2200  deg.  C.  Here  the  distances  measured  horizontally 
represent  wave-lengths,  the  same  as  in  the  photograph,  but  the  intensity  of  each  wave- 
length is  represented  by  the  vertical  height  of  the  corresponding  point  on  the  curve,  in- 
stead of  by  the  blackness. 

The  spectrum  of  X-rays,  which  are  given  out  by  a  solid  metal  when  it  is  struck  by 
high  speed  electrons,  is,  in  appearance,  a  combination  of  incandescent  solid  and  vapor, 
that  is,  it  has  both  the  strong  continuous  spectrum  and  strong  lines.  The  wave-lengths 
are,  of  course,  much  shorter  than  those  of  ordinary  light. 

THE  MECHANISM  OF  RADIATION  AND  REFLECTION 

A  ray  of  light  consists  of  trains  of  waves  sent  out  by  the  vibrating  electrons  in  the 
luminous  body,  one  train  from  each  electron,  just  as  a  train  of  water  waves  is  sent  out 
by  any  vibrating  object  in  water,  or  sound  waves  by  a  vibrating  tuning-fork  in  the  air. 


Fig.  1.     Diffraction  Grating  Spectrometer 

In  the  case  of  light  the  waves  consist  of  electric  force  instead  of  water  or  air,  but  in  all 
other  respects  they  resemble  water  waves  very  closely.  Picture  an  electron  endowed 
with  eyes  standing  at  a  point  in  the  path  of  the  ray  of  light.  The  electron  will  observe 
that  the  electric  force  at  the  point  where  he  is  standing  is  now  upward,  that  is,  in  such 
a  direction  that  an  electrically  charged  body  would  be  pulled  upward  by  it,  now  down- 
ward, now  up,  now  down,  etc.,  in  regular  sequence,  with  a  periodicity  which  is  called 
the  "frequency"  of  the  light;  and  if,  at  the  instant  when  a  crest  is  passing  him,  that  is, 
when  the  electric  force  is  upward  and  at  maximum  intensity,  he  looks  backward  to  the 
next  approaching  crest,  the  distance  to  this  crest  is  a  wave-length  of  the  light.  The 
electron  will  also  report  that  the  electric  force,  instead  of  being  alternately  up  and  down, 
may  be  to  the  right  and  left  respectively,  or  in  any  other  direction  at  right  angles  to  that 
in  which  the  rays  are  travelling ;  and  that  the  electric  force  is  always  accompanied  by  a 
magnetic  force  at  right  angles  to  it.  But  being  electrical,  he  will  be  chiefly  concerned 
with  the  electric  force,  and  since  reflection  depends  on  electrons,  our  interests  are  iden- 
tical with  his' 

170 


We  may  now  give  a  picture  of  reflection.  The  observing  electron  of  the  preceding 
paragraph  will  experience  the  pull  of  the  electric  force,  and,  unless  he  is  very  firmly 
anchored, — and  we  have  good  evidence  that  most  of  the  electrons  in  matter  are  only 
loosely  held  to  their  positions  in  the  atoms — will  soon  find  himself  riding  on  the  wave, 
moving  up  and  down  in  synchronism  with  it.  And  being  electrical,  he  cannot  oscillate 
in  this  way  without  sending  out  waves  of  electric  force,  like  the  electrons  in  hot  bodies. 
These  secondary  waves  constitute  reflected  light.  The  reflected  rays  are  to  be  looked 
upon  as  new  rays,  not  the  primary  ones  turned  back,  although  it  is,  of  course,  the  same 
energy,  slightly  diminished,  that  appears  in  these  reflected  rays.  A  good  analogy  is  a 
motor-generator  generating  alternating  current  of  the  same  frequency  as  the  primary 
current. 

THE  FORMATION  OF  SPECTRA 

The  picture  of  reflection  just  given  can  now  be  applied  to  explain  the  separation  of 
different  wave  lengths  of  either  X-rays  or  ordinary  light  into  a  spectrum.  To  obtain 


Fig.  2.     The  Crystal  X-ray  Spectrometer 

the  spectrum  of  a  beam  of  ordinary  light  we  select  a  small  portion  of  it  by  means  of  a 
narrow  slit  5,  Fig.  1 ,  make  its  ray^travel  in  parallel  lines  by  a  lens  L,  and  let  them  fall  on 
a  grating  M-N.  The  grating  consists  of  a  plate  of  glass  or  metal  ruled  with  a  large  num- 
ber of  fine,  parallel  grooves.  Those  electrons  in  the  grating  surface  upon  which  the 
light  falls  are  set  into  oscillation  by  it  and  each  one  becomes  a  new  source  of  light 
waves  which  it  sends  out  in  all  directions. 

In  order  to  obtain  the  spectrum  of  our  source  of  light  we  have  only  to  add  up  these 
secondary  wavelets,  each  with  its  proper  phase.  If  the  wavelets  sent  out  by  two 
electrons,  a  and  b,  Fig.  1,  arrive  at  a  point  P  in  exactly  opposite  phase,  the  electric  force 
due  to  one  will  always  be  downward  when  that  due  to  the  other  is  upward,  and  of  the 
same  magnitude,  i.e.,  we  shall  have  at  every  instant  two  equal  and  opposite  electric 

171 


forces  at  that  point.  The  resultant  electric  force  will  therefore  be  zero  continuously, 
and  if  the  eye  be  placed  there  the  electrons  in  the  retina  that  cause  the  sensation  of 
sight  will  not  be  set  into  vibration.  If,  however,  the  waves  arrive  at  P  in  the  same 
phase,  their  electric  forces  add,  and  the  light  is  doubled. 

The  wavelets  from  the  grooved  portions  need  not  be  considered,  as  their  phase  rela- 
tions are  so  irregular  that  their  resultant  is  always  small.  The  locus  of  points  at  which 
the  wavelets  from  the  plane  areas  a,  b,  c,  etc.,  arrive  in  the  same  phase  may  be  found 
from  geometry  as  follows: 

The  forced  oscillation  of  the  electrons  in  the  metal  surface  follows  exactly  the  exciting 
wave,  so  that  the  secondary  wavelets,  at  the  moment  of  starting  out,  are  in  phase  with 
the  primary.  The  relative  phase  of  the  different  rays  at  P  depends,  therefore,  only  on 
the  distances  that  the  rays  have  to  travel  from  5  to  P.  A  part  of  the  distance,  that  from 
S  and  P  to  planes  l^o  and  W\  perpendicular  to  the  direction  of  the  rays  respectively,  we 
know  to  be  the  same  for  all  rays  on  account  of  the  action  of  the  lens,  so  that  the  dis- 
tances to  be  compared  are  from  W0  to  W\.  It  is  evident  at  once  from  the  figure  that  this 
distance  is  exactly  the  same  for  all  rays  provided  the  "angle  of  incidence,"  SbM  is 
equal  to  the  "angle  of  reflection "  PbN .  This  gives  the  law  of  ordinary  reflection,  and  is 
true  for  all  wave-lengths,  and  independently  of  whether  the  surface  M  N  is  continuous 
or  broken  by  scratches.  If  the  surface  is  continuous  this  is  the  only  direction  in  which 
the  secondary  waves  are  all  in  phase,  that  is,  this  is  the  only  direction  in  which  light 
is  reflected.  But  if  the  surface  is  broken  by  grooves  equally  spaced,  there  is  another  di- 
rection P'  in  which  the  optical  distance  S-P'  for  rays  from  consecutive  plane  areas 
(a,  6,  etc.)  differs  by  just  one  wave-length,  so  that  the  wavelets  from  a  arrive  just  one 
wave-length  ahead  of  those  from  b,  those  from  b  one  wave-length  ahead  of  those  from 
c,  etc.  They  will  thus  be  in  phase  atP'  and  light  of  this  wave-length  will  be  intense  at  P'. 
For  a  different  wave-length  the  wavelets  will  not  be  in  phase  at  P',  but  will  be  at  some 
other  point  P".  Thus  the  different  wave-lengths  will  be  separated  and  form  a  spectrum. 

It  is  also  possible  for  the  wavelets  from  a  to  arrive  at  some  point  exactly  two,  three, 
or  four  wave-lengths  ahead  of  those  from  b,  and  so  be  in  phase.  The  spectra  thus 
formed  are  called  spectra  of  the  "second  order,"  "third  order,"  etc.  A  photograph  of 
the  complete  spectrum  will  generally  contain  several  orders,  some  of  them  overlapping 
each  other. 

In  the  case  of  X-rays  the  picture  is  still  simpler;  for  the  wave-lengths  are  so  short 
that  we  are  able  to  use  for  a  grating  a  natural  crystal  such  as  rock  salt,  in  which  the  indi- 
vidual atoms  take  the  place  of  the  little  faces  a,  b,  c,  of  Fig.  1 .  Crystallography  teaches 
that  the  atoms  in  crystals  are  arranged  in  regular,  equidistant  planes,  and  Prof.  Bragg 
and  his  son  have  been  able,  by  means  of  X-ray  spectra,  not  only  to  confirm  this 
hypothesis  but  to  find  the  exact  positions  of  the  atoms.  They  find  that  the  atoms  in 
each  plane  are  equally  spaced  in  parallel  rows.  These  rows  of  atoms  correspond  to  the 
narrow  plane  surfaces  between  grooves  in  the  diffraction  grading.  The  only  difference 
between  the  crystal  and  the  grating  is  that  the  X-rays  penetrate  several  thousand 
planes  deep,  so  that  the  crystal  is  like  a  pile  of  semi-transparent  gratings,  all  equidistant 
and  with  their  lines  parallel. 

The  use  of  the  crystal  as  a  grating  is  shown  graphically  in  Fig.  2,  where  F  represents 
the  hot  filament  cathode  of  the  X-ray  tube,  T  the  target,  C  the  crystal,  and  P  the  photo- 
graphic plate.  The  electrons  of  the  " cathode  ray"  stream  fall  upon  the  target  and  set 
the  electrons  of  the  atoms  in  its  surface  into  violent  vibration.  It  is  easy  to  conceive 
how  the  frequency  of  vibration  caused  by  one  of  these  blows,  from  an  electron  moving 
with  half  the  velocity  of  light,  should  be  much  higher  than  that  caused  by  a  bump  from 
another  atom,  such  as  gives  rise  to  the  visible  light  of  a  hot  body. 

172 


These  vibrating  electrons  of  the  target  send  out  the  high  frequency  electric  waves 
which  we  call  X-rays.  They  travel  out  in  all  directions,  and  a  portion  of  them,  passing 
through  the  narrow  slit  5,  fall  on  the  crystal  C,  and  cause  the  electrons  in  its  atoms  to 
vibrate  and  radiate  secondary  wavelets.  These  secondary  wavelets  then  travel  to  the 
photographic  plate  P  and  there  reinforce  or  annul  each  other  according  to  their  phase 
relations,  as  in  the  case  of  the  visible  spectrum  already  discussed. 

To  find  the  proper  phase  relations  it  is  best  to  proceed  in  two  steps,  first  considering 
the  atoms  in  a  single  plane,  and  then  the  relation  of  the  planes  to  each  other.  For  a 
single  plane  the  phase  relations  are  exactly  the  same  as  for  the  grating,  since  the  plane 
with  is  rows  of  atoms  acts  just  like  a  grating.  We  need,  for  the  present  purpose,  only 
the  first  relation  deduced  above,  namely,  that  the  wavelets  from  all  the  atoms  in 
the  plane  will  be  in  phase  with  each  other  provided  the  "angle  of  incidence"  of  the 
rays  on  the  plane  is  equal  to  the  so  called  "angle  ot  reflection,"  the  angle  at  which 
that  part  of  the  secondary  rays  which  we  are  considering  leaves  the  crystal.  Any  one 
atom  in  the  plane  may  therefore  represent  the  phase  of  all  of  them,  provided  we  keep 
the  angles  of  incidence  and  reflection  equal. 


\ 


Fig.  3.     Visible    Spectrum    of 
Tungsten     Vapor.       (Wave 
Lengths  in  Angstrom  Units) 


WOO  OOOO  I5OOO  10000  2X00  300OO  35000 

Waft  Lemjtfi 

Fig.  4.      Spectrum  of  Incandescent  Solid  Tungsten 


The  second  part  of  the  problem  is  to  find  under  what  conditions  the  wavelets  from 
the  atoms  in  the  first  plane  are  in  phase  with  those  from  the  second,  etc.  Let  us  take 
as  representative  atoms  from  the  different  planes,  those  which  lie  in  a  straight  line 
parallel  to  the  primary  beam,  as  ai,  61,  c\,  etc.,  Fig.  2.  (If  necessary  the  planes  may  be 
imagined  to  slide  over  each  other  until  these  atoms  are  in  line.)  The  primary  wave 
s  a  b  c  reaches  b  later  than  a,  so  that  the  phase  of  oscillation  of  the  electrons  of  6,  and 
hence  of  the  wavelets  which  they  send  out,  will  be  behind  those  of  a.  The  wavelets  from 
b  also  have  a  greater  distance  to  travel  to  P  than  those  from  a,  so  that  they  will  be  still 
more  behind  in  phase  when  they  arrive  at  P.  If,  however,  they  are  a  whole  wave- 
length, or  any  whole  number  of  wave-lengths  behind,  they  will  be  in  phase,  and  the 
electric  forces  of  the  two  will  add.  Exactly  the  same  relation  will  exist  between  the 
wavelets  from  c  and  b,  d  and  c,  etc.,  since  the  planes  are  equidistant.  Hence  the  wave- 
lets from  all  the  atoms  in  the  crystal  will  be  in  phase  at  P  when  two  conditions  are  ful- 


173 


filled :  (1)  the  angle  of  incidence  must  be  equal  to  the  angle  of  reflection;  (2)  this  angle 
must  be  such  that  the  wavelets  from  successive  planes  differ  in  phase  by  some  integral 
number  of  wave-lengths.  We  then  obtain  a  registration  on  the  photographic  plate  at 
P,  of  one  wave-length  of  the  primary  beam.  For  a  different  wave-length  the  wavelets 
will  be  in  phase  at  some  adjacent  point  P'  on  the  photographic  plate,  provided  we 
rotate  the  crystal  until  the  angles  of  incidence  and  reflection  are  again  equal,  and  of  the 
proper  value  for  this  new  wave  length.  Thus  by  continuous  rotation  of  the  crystal, 
which  is  accomplished  by  a  motor  and  worm  gear,  all  the  wave-lengths  in  the  beam  are 
successively  registered. 

According  to  the  second  condition  given  above,  the  same  wave-length  may  be 
registered  at  several  different  positions  on  the  plate,  corresponding  to  phase  differences 
of  one,  two,  three,  etc.,  wave-lengths  between  wavelets  from  consecutive  planes. 
Hence  if  the  crystal  is  rotated  far  enough,-  several  complete  spectra  will  be  obtained, 
called  respectively  the  first,  second,  third,  etc.,  order  spectra.  The  intensity  of  the 


Fig.  5.     Complete  X-ray  Spectrum  of  Tungsten  at  100,000  Volts 


higher  orders  is  very  small,  so  that  usually  not  more  than  three  orders  are  visible.  Fig. 
6  shows  the  so-called  "K"  lines  of  tungsten  in  two  orders,  and  Fig.  5  in  three  orders. 
The  photographic  plate  gives  the  correct  values  of  the  wave-lengths  present  in  the 
beam,  but  not  the  intensity.  In  order  to  obtain  this  we  make  use  of  the  fact  that  when 
X-rays  pass  through  a  gas  they  make  it  electrically  conducting.  Hence  if  the  rays  are 
allowed  to  enter  an  "ionization  chamber,"  which  consists  simply  of  two  oppositely 
charged  plates,  a  current  will  flow  through  the  gas  between  the  plates.  This  current  can 
be  measured  by  a  sensitive  electrometer  and  is  proportional,  if  the  gas  is  dense  enough  to 
absorb  nearly  all  the  rays,  to  the  intensity  of  the  rays.  Thus  by  putting  the  ionization 
chamber  in  place  of  the  photographic  plate,  and  reading  the  electrometer  at  regular  in- 
tervals while  the  crystal  is  being  rotated,  one  obtains  the  intensities  of  all  the  wave- 
lengths in  the  spectrum.  The  spectrum  shown  in  Fig.  7  was  obtained  in  this  way. 

174 


DESCRIPTION  OF  THE  SPECTRUM 

The  X-ray  spectrum  of  tunsten,  obtained  as  described  above,  is  shown  in  Figs.  5,  (i 
and  7.  It  consists  of  a  "continuous  spectrum"  extending  over  four  octaves  (from  the 
wave-length  X  =  0.12X  1CT8  cm.  toX  =  2X  1CT8  cm.),  and  16  lines.  Four  of  these  lines,  on 
the  short  wave-length  end  of  the  spectrum,  are  very  close  together  and  are  known  as 
the  K  series.  They  are  usually  designated  as  a1,  a,  ft  and 7.  In  Fig.  5,  these  four  lines 
appear  as  only  two,  the  "a  doublet"  appearing  as  a  single  line,  and  the  ft  and  7  lines 
being  likewise  too  close  to  appear  separately.  The  other  12  lines  form  another  group, 
with  wave-lengths  nearly  ten  times  those  of  the  K  series,  and  are  known  as  the  L  series. 

Fig.  6  shows  a  photograph  taken  in  the  manner  described  above,  with  a  Coolidge 
tube  having  a  tungsten  target,  running  at  100,000  volts  and  1.2  milliamperes.  The 
rock  salt  crystal  (C,  Fig.  2),  was  40  cm.  from  the  target  T  and  56  cm.  from  the  photo- 
graphic plate,  and  was  kept  in  continuous  rotation  during  the  four-hour  exposure. 

The  photograph  shows  three  of  the  four  "K"  lines  of  tungsten,  the  a  doublet  and 
the  strong  ft  line,  :n  the  first  and  second  orders  (marked  with  subscripts  1  and  2  re- 
spectively). The  7  line,  which  is  just  to  the  left  of  the  ft  line,  is  too  weak  to  show.  The 
wave-lengths  of  these  lines  are  0.212,  0.208,  and  0.185  Angstrom  units*  for  the  two 
a  lines  and  the  ft  line  respectively.  The  wave-length  of  the  ft  line,  which  is  the  shortest 

line  that  has  been  observed  in  the  tungsten  X-ray  spectrum,  is  a  little  less  than 

10,000 

of  the  wave-lenth  of  the  shortest  ultraviolet  line  (X  =  2700  Angstroms)  that  has  been 
found  in  the  spark  spectrum  of  tungsten  vapor. 

Fig.  5  is  taken  under  the  same  conditions  of  current  and  voltage  as  Fig.  6,  but  the 
photographic  plate  was  only  one-third  as  far,  19  cm.,  from  the  crystal  and  the  crystal 
was  rotated  through  a  larger  angle  so  as  to  obtain  a  larger  portion  of  the. spectrum. 
The  time  of  exposure  was  six  hours.  On  this  photograph  C  is  the  undeviated  primary 
beam  of  rays  which  has  passed  straight  through  the  crystal,  and  marks  the  zero. line 
from  which  to  measure  the  lines  on  the  spectrum.  The  wave-lengths  of  these  lines 
are  approximately  proportional  to  their  distance  from  this  zero  line,  except  for  the  lines 
of  2nd  and  3rd  order,  whose  distances  must  be  divided  by  2  and  3  respectively. 

The  wave-length  marked  X0  at  the  extreme  left  of  the  photograph,  the  shortest  wave- 
length present  in  the  spectrum,  is  connected  in  a  very  interesting  way  with  the  velocity 
of  the  electrons  which  impinge  upon  the  target  in  the  X-ray  tube,  and  produce  the  rays. 
If  we  speak  in  terms  of  frequency  instead  of  wave-length,  the  frequency  of  this  limiting 
wave-length  multiplied  by  Planck's  universal  constant  "h,"  the  so-called  "quantum," 
is  exactly  equal  to  the  kinetic  energy  of  the  impinging  electron.  This  relation  has  been 
checked  over  the  whole  range  of  voltage  from  20,000  to  100,000  volts,  and  is  more  than 
a  coincidence.  It  is  another  of  the  striking  mathematical  relations  which  the  "  quantum 
theory  "  f  has  brought  to  light,  and  which,  though  not  at  present  understood,  must  have 
an  extremely  intimate  connection  with  the  mechanism  of  atomic  structure. 

The  lines  markeda:i,a2,a3.  and  fti,  /32,  ft3  are  the  first,  second  and  third  orders  respect- 
ively of  the  a  and  ft  lines  of  the  "K"  series.  The  two  a  lines  show  separately  in  the 
second  order,  but  not  in  the  first.  The  7  line  is  not  visible. 

The  band  whose  edge  is  marked  Agi  is  an  absorption  band  of  the  silver  in  the  photo- 
graphic plate.  For  all  wave-lengths  shorter  than  this  wave-length  Agi  (0.485  Ang- 
stroms), silver  has  an  especially  strong  absorption.  Since  photographic  action  depends 


A  i 

ngstrom  unit  nnncm--  >s  l^e  standard  unit  for  expressing  the  wave-lengths  of  visible  light,  and  is  used 

lUU.UUU.UUU  0 

here  for  the  sake  of  comparison.     The  wave-length  of  ordinary  green  light  is  5000  Angstrom  units. 

t  For  a  brief  review  of  the  quantum  theory  see  Dushman,  G.  E.  Rev.,  Sept.,  1914. 

175 


upon  the  amount  of  light  which  the  sensitive  film  absorbs,  and  since  about  99  per  cent  of 
the  energy  of  X  light  goes  through  the  plate  without  absorption,  it  is  evident  that  an  in- 
crease in  the  absorbing  power  of  the  silver  will  cause  a  large  increase  in  blackening. 
The  band  Agz  is  also  due  to  the  silver,  and  is  caused  by  the  "  second  order  "  reflection 
of  these  same  wave-lengths,  striking  the  plate  this  time  at  a  point  twice  as  far  from  the 


Fig.  6.     X-ray  Spectrum  of  Tungsten — The  "  K  "  Lines 


central  line  C.  In  the  same  way  the  band  Br  is  due  to  the  special  absorption,  by  the 
bromine  atoms  in  the  silver  bromide  of  the  photographic  plate,  of  all  wave-lengths 
shorter^than  0.918  Angstroms. 


.7         .a 
Fig.  7.     X-ray  Spectrum  of  Tungsten  at  100,000  Volts,  Obtained  by  the  lonization  Chamber 

There  is  one  other  absorption  band,  which  shows  clearly  on  the  original  photograph, 
just  to  the  left  of  Br.  It  is  due  to  the  special  absorption,  by  the  minute  trace  of 
rubidium  in  the  glass  of  the  X-ray  bulb,  of  all  wave-lengths  shorter  than  0.814  Ang- 
stroms. In  this  case  special  absorption  means  a  loss  of  light  to  the  photographic  plate, 
hence  the  spectrum  to  the  left  of  Rb  is  less  black  than  that  to  the  right.  The  absorption 

176 


bands  due  to  the  other  constituents  of  the  glass  fall  too  far  to  the  right  to  show  on  the 
photograph. 

All  known  elements  have  these  X-ray  absorption  bands,  and  their  positions  are 
much  more  regular  and  more  simply  related  to  the  material  than  are  the  bands  in  the 
visible  spectrum.  As  far  as  known  every  element  has  two  absorption  bands  for  X-rays, 
one  beginning  at  a  wave-length  just  beyond  its  K  series  on  the  short  wave-length  side, 
the  other  just  beyond  its  L  series.  The  wave-lengths  at  which  each  of  these  bands  be- 
gin, for  the  different  elements,  are  very  nearly  proportional,  inversely,  to  the  squares  of 
the  "  atomic  numbers  "  of  the  respective  elements.*  The  physical  meaning  of  this  rela- 
tion also,  like  the  quantum,  is  not  yet  known.  Its  simplicity  and  exactness  give  it 
significance. 

The  rest  of  the  lines,  those  to  the  right  of  Ag%,  all  belong  to  the  "  L  "  series.  For  con- 
venience of  identification  they  are  lettered  a-k.  Their  wave-lengths  range  from  1.47 
Angstroms  for  "a"  to  1.033  for  "k."  They  are  all  in  the  first  order. 

For  the  purpose  of  comparison  the  spectrum  of  tungsten  vapor,  made  luminous  by 
an  electric  spark,  is  shown  in  Fig.  3.  The  lines  do  not  show  very  clearly  because  they 
are  so  numerous.  Compared  with  the  complexity  of  these  visible  spectra,  some  of  which 
contain  as  many  as  60,000  lines,  the  X-ray  spectra  are  strikingly  simple.  This  simplic- 
ity makes  the  X-ray  spectra  especially  useful,  both  for  scientific  investigation  and  as  a 
means  of  chemical  analysis. 

The  continuous  spectrum,  which  appears  as  a  continuous  background  in  Figs.  5  and 
G,  is  shown  graphically  in  Fig.  7.  This  was  obtained  by  the  use  of  an  ionization  chamber 
and  electrometer,  in  place  of  the  photographic  plate,  as  explained  above.  The  current 
and  voltage  were  the  same  as  for  Figs.  5  and  6,  viz.,  100,000  volts  and  1.2  milliamperes. 
The  ordinates  of  points  on  the  curve  give  the  intensity  of  the  corresponding  wave- 
lengths, whose  values,  in  Angstroms,  are  given  by  the  abscissas.  The  circles  mark  the 
experimental  measurements  as  read  from  the  electrometer. 

The  curve  shows  clearly  the  repetition  in  the  first,  second  and  third  orders  of  the  a 
and  /3  lines  shown  in  the  photographs  of  Figs.  5  and  6.  It  also  shows  the  relative  inten- 
sity of  the  lines  as  compared  with  the  continuous  spectrum  upon  which  they  are  super- 
imposed. The  continuous  spectrum  is,  like  the  lines,  present  in  all  three  orders,  so  that 
to  obtain  the  true  relative  intensity  of  the  different  wave-lengths  in  the  beam  it  is  neces- 
sary to  separate  these  different  orders.  This  has  been  done  for  a  lower  voltage,  70,000, 
and  the  resulting  values  are  shown  in  the  dotted  curve,  Fig.  4.  Here  the  K  lines  are  ab- 
sent, as  the  voltage,  70,000,  was  not  high  enough  to  excite  them.  For  comparison  the 
visible  and  infra-red  spectrum  of  incandescent  tungsten  at  2200  deg.  C.,  which  is  ap- 
proximately the  temperature  of  the  filament  of  a  Mazda  lamp,  is  given  in  Fig.  4.  The 
shaded  portion  represents  the  visible  part.  If  the  wave-lengths  in  Fig.  4  were  all  re- 
duced ten  thousand  fold,  it  would  conincide  very  nearly  with  the  dotted  curve  in 
Fig.  7. 

*  The  atomic  number  of  an  element  is  the  number  of  its  position  in  a  table  arranged  according  to  atomic  weight,  be- 
ginning with  hydrogen  equal  to  one.  helium  two,  etc.  It  has  been  found  to  be  more  intimately  connected  with  the  chemical 
properties  of  the  atom  than  the  atomic  weight,  and  is  probably  very  closely  related  to  the  number  of  electrons  in  the  atom. 


177 


A  NEW  METHOD  OF  X-RAY  CRYSTAL  ANALYSIS* 


BY  A.  W.  HULL 

The  beautiful  methods  of  crystal  analysis  that  have  been  developed  by  Laue  and  the 
Braggs  are  applicable  only  to  individual  crystals  of  appreciable  size,  reasonably  free 
from  twinning  and  distortion,  and  sufficiently  developed  to  allow  the  determination  of 
the  direction  of  their  axes.  For  the  majority  of  substances,  especially  the  elementary 
ones,  such  crystals  cannot  be  found  in  nature  or  in  ordinary  technical  products,  and 
their  growth  is  difficult  and  time-consuming. 

The  method  described  below  is  a  modification  of  the  Bragg  method,  and  is  ap- 
plicable to  all  crystalline  substances.    The  quantity  of  material  required  is  preferably 
0.005  c.c.,  but  one  tenth  of  this  amount  is  sufficient.    Extreme  purity  of  material  is  not 
required,  and  a  large  admixture  of  (uncombined) 
foreign  material,  twenty  or  even  fifty  per  cent,  is 
allowable  provided  it  is  amorphous  or  of  known 
crystalline  structure. 

OUTLINE  OF  METHOD 

The  method  consists  in  sending  a  narrow 
beam  of  monochromatic  X-rays  (Fig.  2)  through 
a  disordered  mass  of  small  crystals  of  the  sub- 
stance to  be  investigated,  and  photographing 
the  diffraction  pattern  produced.  Disorder,  as 
regards  orientation  of  the  small  crystals,  is 
essential.  It  is  attained  by  reducing  the  sub- 
stance to  as  finely  divided  form  as  practicable, 
placing  it  in  a  thin-walled  tube  of  glass  or  other 
amorphous  material,  and  keeping  it  in  con- 
tinuous rotation  during  the  exposure.2  If  the 
particles  are  too  large,  or  are  needle-shaped  or 
lamellar,  so  that  they  tend  to  assume  a  definite 

orientation,  they  are  frequently  stirred.  In  this  way  it  is  assured  that  the  average 
orientation  of  the  little  crystals  during  the  long  exposure  is  a  random  one.  At  any 
given  instant  there  will  be  a  certain  number  of  crystals  whose  100  planes  make  the 
proper  angle  with  the  X-ray  beam  to  reflect  the  particular  wave-length  used,  a  certain 
number  of  others  whose  111  planes  are  at  the  angle  appropriate  for  reflection  by  these 
planes,  and  so  for  every  possible  plane  that  belongs  to  the  crystal  system  represented. 
Each  of  these  little  groups  will  contain  the  same  number  of  little  crystals,  provided  the 
distribution  is  truly  random,  and  the  total  number  of  crystals  sufficiently  large.  This 
condition  is  very  nearly  realized  in  the  case  of  fine  powders,  and  may,  by  sufficient 
rotation  and  stirring,  always  be  realized  for  the  average  orientation  during  the  whole 


Fig.  2 


*  Copyright,  1917,  by  the  American  Physical  Society. 

of  r1«>«-rir.tir>n  r>f  tViis  m«>ttir>/1  nrae  oi'iror.  H»f«-,r»  tVi/>  AmonV-an  Physical  Society  in  October,  1916,  and  published  in 


*  Copyright,  1917,  by  the  American  Physical  Society. 

>  A  brief  description  of  this  method  was  given  before  the  American  -  ..j^~>  ~^ 
this  journal  for  January,  1917. 

•  If  the  powder  is  fine,  rotation  i  snot  necessary  unlessgreat  precision  is  desired 
eter,  or  less,  the  pattern  generally  appears  quite  uniform  without  rotation. 


With  crystal  grains  0.01  cm.  in  diam- 


179 


exposure;  that  is,  there  will  be,  on  the  average,  as  many  cubic  centimeters  of  crystals 
reflecting  from  their  100  planes  as  there  are  cubic  centimeters  reflecting  from  111,  210, 
or  any  other  plane.  This  is  true  for  every  possible  plane  in  the  crystal. 

The  diffraction  pattern  should  contain,  therefore,  reflections  from  every  possible 
plane  in  the  crystal,  or  as  many  of  these  as  fall  within  the  limits  of  the  photographic 
plate.  Fig.  1,  Plate  1,  shows  the  pattern  given  by  aluminium  when  illuminated  by  a 
small  circular  beam  of  nearly  monochromatic  rays  from  a  molybdenum  tube.  The 
exposure  was  nine  hours,  with  37  milliamperes  at  30,000  volts,  and  crystal  powder  15 
cm.  from  the  target  and  5.9  cm.  from  photographic  plate.  The  faintness  of  the  vertical 
portions  of  the  circles  is  due  to  the  cylindrical  form  in  which  the  powder  was  mounted, 
causing  greater  absorption  of  rays  scattered  in  the  vertical  plane.  Patterns  containing 
many  more  lines  are  shown  in  Figs.  6-10,  where  the  diaphragm  limiting  the  beam  was  a 
slit  instead  of  a  circular  aperture,  and  the  pattern  was  received  on  a  photographic  film 
bent  in  the  arc  of  a  circle. 

The  number  of  possible  planes  in  any  crystal  system  is  infinite.  Hence  if  equal  re- 
flecting opportunity  meant  equal  reflected  energy,  it  would  follow  that  the  energy  re- 
flected by  each  system  of  planes  must  be  an  infinitesimal  fraction  of  the  primary  beam, 
and  hence  could  produce  no  individual  photographic  effect.  It  is  easily  seen,  however, 
that  only  those  planes  whose  distance  apart  is  greater  than  X/2,  where  X  is  the  wave- 
length of  the  incident  rays,  can  reflect  any  energy  at  all.  Planes  whose  distance  apart 
if  less  than  this  cannot  have,  in  any  direction,  except  that  of  the  incident  beam,  equality 
of  phase  of  the  wavelets  diffracted  by  electrons  in  consecutive  planes.  Hence  the 
resultant  amplitude  associated  with  any  such  plane  is  very  small,  and  would  be  identi- 
cally zero  for  a  perfect  lattice  and  sufficiently  large  number  of  planes.  The  total  scat- 
tered energy  is,  therefore,  divided  among  a  finite  number  of  planes,  each  of  which  pro- 
duces upon  the  photographic  plate  a  linear  image  of  the  source  (cf.  Fig.  1).  The  total 
possible  number  of  these  lines  depends  upon  the  crystal  structure  and  the  wave-length . 
For  diamond,  with  the  wave-length  of  the  KA  doublet  of  molybdenum,  X  =  0.712,  the 
total  number  of  lines  is  27.  All  of  these  are  present  in  the  photograph  shown  in  Fig.  12. 
For  the  rhodium  doublet,  X  =  0.617,  the  total  number  is  30;  for  the  tungsten  doublet, 
X  =  0.212,  it  is  more  than  100;  while  the  iron  doublet,  X=  1.93,  can  be  reflected  by  only 
three  sets  of  diamond  planes,  the  octahedral  (111),  rhombic  dodecahedral  (110),  and  the 
trapezohedral  (311).  The  diffraction  pattern  in  this  case  would  consist,  therefore,  of 
but  three  lines. 

The  positions  of  these  lines,  in  terms  of  their  angular  deviation  from  the  central 
beam,  are  completely  determined  by  the  spacing  of  the  corresponding  planes,  according 
to  the  classic  equation  n\  =  2d  sin  6,  where  6  is  the  angle  between  the  incident  ray  and 
the  plane,  hence  26  is  the  angular  deviation,  d  the  distance  between  consecutive  planes, 
X  the  wave-length  of  the  incident  rays,  and  n  the  order  of  the  reflection.  The  calcula- 
tion of  these  positions  is  discussed  in  detail  below. 

The  relative  intensity  of  the  lines,  when  corrected  for  temperature,  angle,  and  the 
number  of  co-operating  planes,  depends  only  upon  the  space  distribution  of  the  electrons 
of  which  the  atoms  are  composed.  Most  of  these  electrons  are  so  strongly  bound  to 
their  atoms  that  their  positions  can  probably  be  completely  specified  by  the  positions  of 
the  atomic  nuclei  and  the  characteristic  structure  of  the  atom.  Experiments  are  in 
progress  to  determine  such  a  structure  for  some  of  the  simpler  atoms.  A  few  of  the 
electrons,  however,  are  so  influenced  by  the  proximity  of  other  atoms,  that  their  posi- 
tion will  depend  much  on  the  crystal  structure  and  state  of  combination  of  the  sub- 
stance. There  is  also  good  reason  to  believe  that  certain  electrons  are  really  free,  in 

180 


Fig.  7b.     Silicon  Steel 


Fig.  1.     Aluminium 


Fig.  5.     Tungsten  X-Ray  Spectrum 


Fig.  8.     Silicon 


Fig.  9.     Aluminum 


Fig.  6.     Iron 


Fig.  10.     Magnesium 


Fig.  11.     Graphite 


Fig.  7a.     Silicon  Steel 


Fig.  12.     Diamond 


181 


that  they  belong  to  no  atom,  but  occupy  definite  spaces  in  the  lattice,  as  though  they 
were  atoms. 

With  elements  of  high  atomic  weight,  where  each  atom  contains  a  large  number 
of  elctrons,  the  majority  of  these  electrons  must  be  quite  close  to  the  nucleus,  so  that 
the  intensity  of  the  lines  will  depend  primarily  upon  the  position  of  the  nuclei  relative 
to  their  planes,  and  only  slightly  upon  the  characteristic  structure  of  the  atom  and  the 
position  of  valence  and  free  electrons.  With  these  substances,  therefore,  the  relative 
intensity  of  the  lines  gives  direct  evidence  regarding  the  positions  of  the  atoms,  and  may 
be  used,  in  the  manner  described  by  the  Braggs,1  for  the  determination  of  crystal 
structure.  The  powder  photographs  have  an  advantage,  in  this  respect,  over  ionization- 
chamber  measurements,  in  that  the  intensities  of  reflection  from  different  planes,  as  well 
as  different  orders,  are  directly  comparable,  which  is  not  true  of  ionization-chamber 
measurements  unless  the  crystal  is  very  large  and  may  be  ground  for  each  plane. 

In  the  case  of  light  substances,  on  the  other  hand,  the  intensities  depend  very  much 
on  the  internal  structure  of  the  atoms,  and  unless  this  structure  is  known  or  postulated, 
but  little  weight  should  be  given  to  intensity  in  determining  the  crystal  structure.  Much 
evidence  for  the  structure  of  these  elements  may  be  obtained,  however,  from  the  ob- 
servation of  the  position  of  a  large  number  of  lines,  and  this  evidence  will  generally  be 
found  sufficient.  The  examples  given  at  the  end  of  this  paper  are  all  elements  of  low 
atomic  weight,  and  the  analysis  given  is  based  entirely  on  the  position  of  the  lines.  The 
photographs  used  for  the  analysis  are  preliminary  ones,  taken  with  very  crude  experi- 
mental arrangements,  and  yet  in  every  case,  except  one,  the  evidence  is  sufficient. 

The  method  of  measuring  and  interpreting  intensity  will  form  the  subject  of  a  future 
paper. 

EXPERIMENTAL  ARRANGEMENT 

The  arrangement  of  apparatus  is  shown  in  Fig.  2.  The  X-ray  tube  is  completely 
enclosed  in  a  very  tightly  built  lead  box.  If  a  tungsten  target  is  to  be  used  this  box 
should  be  of  j^-inch  lead,  with  an  extra  34 -inch  on  the  side  facing  the  photographic 
plate.  If  a  rhodium  or  molybdenum  target  is  used  Y%  inch  on  the  side  toward  the 
photographic  plate,  and  -^  inch  for  the  rest  of  the  box,  is  sufficient.  The  rays  pass 
through  the  filter  F  and  slits  Si  and  52,  and  fall  upon  the  crystal  substance  C,  by  which 
they  are  diffracted  to  points  pi,  p»,  etc.,  on  the  photographic  plate  P.  The  direct  beam 
is  stopped  by  a  narrow  lead  strip  H,  of  such  thickness  that  the  photographic  image 
produced  by  this  beam  is  within  the  range  of  normal  exposure.  For  a  tungsten  target, 
the  thickness  of  this  strip  should  be  y%  inch ;  for  a  molybdenum  target  about  1/100  inch. 

THE  X-RAY  TUBE 

In  order  to  produce  monochromatic  rays,  it  is  necessary  to  use  a  target  which  gives 
a  characteristic  radiation  of  the  desired  wave-length,  and  to  run  the  tube  at  such  a 
voltage  that  the  radiation  of  this  wave-length  will  be  both  intense  and  capable  of 
isolation  by  filtering. 

The  relation  between  general  and  characteristic  radiation  at  different  voltages  has 
been  investigated,  for  tungsten  and  molybdenum,  by  the  author,2  and,  in  more  detail, 
for  rhodium  by  Webster,  and  platinum  by  Webster  and  Clark.3  The  results  may  be 
summarized  as  follows :  The  characteristic  line  spectra  are  excited  only  when  the  voltage 

hv 
across  the  tube  is  equal  to  or  greater  than  the  value  F  =  — ,  where  h  is  Planck's  con- 

.  t> 

stant,  e  the  charge  of  an  electron,  and  v  the  frequency  corresponding  to  the  short  wave- 
length limit  of  the  series  to  which  the  line  belongs. 


X-Rays  and  Crystal  Structure,  pp.  120  ff. 

Nat.  Acad.  Proc.,  2,  268,  1916. 

Phys.  Rev.,  7,  599,  1916;  Nat.  Acad.  Proc.,  5,  18o,  1917. 

182 


With  increase  of  voltage  above  this  limiting  voltage,  the  intensity  of  the  lines  in- 
creases rapidly,  approximately  proportional  to  the  3/2  power  of  the  excess  of  voltage 
above  the  limiting  value.1  The  following  table  will  show  the  rate  of  increase  for  the  a. 
line  of  the  K  series  of  molybdenum,  as  used  in  the  experiments  described  below.2 

TABLE  I 

Increase  of  Intensity  of  the  Ka  Line  oj  Mo  with  Voltage 


KILOVOLTS 


20. 

i     22. 

24. 

26. 

28. 

30. 

32. 

34 

36. 

40. 

Intensity.  . 

0 

1.25 

2.75 

4.80 

7.30 

9.60 

12.65 

15 

2 

18.5 

23.4 

The  rapid  increase  of  characteristic  radiation  with  voltage  makes  it  desirable  to  use 
as  high  voltage  as  possible.  If  the  voltage  is  too  high,  however,  a  part  of  the  general 
radiation,  whose  maximum  frequency  is  directly  proportional  to  the  voltage,3  becomes 
so  short  that  it  is  impossible  to  separate  it  from  the  characteristic  by  a  selective  filter. 
With  a  molybdenum  target  the  best  working  voltage  is  about  30,000  volts,  with 
tungsten  about  100,000  volts. 

FILTERS 

Although  it  is  impossible  to  produce  truly  monochromatic  radiation  by  filtering,  it 
is  easy  to  obtain  a  spectrum  containing  only  one  line,  and  in  which  the  intensity  of  this 
line  is  more  than  thirty  times  that  of  any  part  of  the  general  radiation.  To  accomplish 
this,  use  is  made  of  the  sudden  increase  in  absorption  of  the  filter  at  the  wave-length 
corresponding  to  the  limit  of  one  of  its  characteristic  series;  that  is,  at  the  wave-length 
which  is  just  short  enough  to  excite  in  the  filter  one  of  its  characteristic  radiations.  A 
filter  is  chosen  whose  K  series  limit4  lies  as  close  as  possible  to  the  desired  wave-length 
on  its  short  wave-length  side.  For  example,  to  isolate  the  K  line  of  molybdenum  whose 
wave-length  is  0.712  A.,  the  most  appropriate  filter  is  zirconium,  the  limit  of  whose  K 
series  is  at  X  =  0.690  A.  The  absorption  coefficient  of  the  filter  is  then  a  minimum  for 
the  wave-length  in  question,  and  increases  rapidly  with  wave-length  in  both  directions; 
on  the  left,  toward  shorter  wave-lengths,  it  jumps  suddenly  by  about  8-fold;  on  the 
right  it  increases  more  slowly,  viz.,  as  the  cube  of  the  wave-length.5 

If  the  longest  wave-length  in  the  series,  which,  fortunately,  in  the  case  of  the  K 
series,  is  the  most  intense,  is  chosen  for  the  monochromatic  ray,  the  eight-fold  increase 
in  absorption  coefficient  will  completely  eliminate  the  other  lines  of  the  series,  while  re- 
ducing the  chosen  line  by  only  one  half.  To  eliminate  the  general  radiation  is  not  so 
easy.  Webster  has  shown6  that  the  intensity  of  the  characteristic  radiation  increases 
more  rapidly  with  voltage  than  that  of  the  neighboring  general  radiation,  so  that  the 
higher  the  voltage  the  more  prominently  the  line  stands  out  above  adjacent  wave- 
lengths, and  this  is  the  only  way  in  which  it  can  be  sharply  separated  from  longer  wave- 
lengths. If  the  voltage  is  too  high,  however,  the  shortest  wave-length  end  of  the  general 
spectrum  becomes  transmissible  by  the  filter,  and  while  its  wave-length  is  far  removed 
from  that  of  the  line  which  is  to  be  isolated,  and  it  can  itself  produce  no  line  image,  yet  its 
integral  effect  produces  a  general  blackening  of  the  plate  that  obscures  the  lines.  Sharp 
limitation  on  the  short  wave-length  side  is  obtained  by  the  selective  action  of  the  filter. 

It  is  necessary,  therefore  to  choose  filter  material,  filter  thickness,  and  voltage,  to 
correspond  to  the  target  used.  For  a  molybdenum  target,  the  filter  should  be  zir- 

1  Webster  and  Clark,  Proc.  Nat.  Acad.,  5,  185,  1917. 

-  The  general  radiation  of  the  same  wave-length  as  the  a  line  is  included  in  these  values. 

3  See  Duane  and  Hunt,  Phy.  Rev.,  6,  619,  and  Hull  Phys.  Rev.,  ~.  156. 

4  A  complete  table  of  wave-lengths  of  series  lines  for  all  elements  thus  far  investigated  is  given  by  Siegbahn,  Jahrb. 
Radioact,  u.  Electronik,  13.  300,  1916. 

5  Hull  and  Rice,  PHYS.  REV.,  8,  326,  1916. 

6  L.  c. 

183 


conium,  and  a  thickness  of  about  0.35  mm.  of  powdered  zircon  is  sufficient1  (see  Fig.  3). 

The  optimum  voltage  is  between  28,000  and  30,000  volts.     For  a  tungsten  target  the 

filter  should  be  ytterbium,  of  a  thickness  of  about  0.15  mm.,  but  this  has  not  yet  been 

tested.    A  filter  of  this  thickness  of  metallic  tungsten  or  tantalum  eliminates  most  of 

the  general  spectrum,  but  leaves  the  /3  doublet  as  well  as  the  a  doublet,  which  is  very 

undesirable  (cf.  Figs.  4  and 
5).  The  optimum  voltage 
is  about  100,000  volts. 

The  effect  of  filtering  on 
the  spectrum  of  a  molybde- 
num target  at28, 000 volts  is 
shown  in  Fig.  3",  which  gives 
the  intensity  of  the  different 
wave-lengths  as  measured 
with  an  ionization  chamber, 
so  constructed  as  to  elimi- 
nate, nearly,  errors  due  to 
incomplete  absorption.2  No 
correction  has  been  made 
for  coefficient  of  reflection 
of  the  (rock  salt)  crystal. 
Fig.  3  The  intensities  of  the  K 

lines  are  too  great   to  be 

shown  on  the  figure,  the  a  line  being  four  times  and  the  line  /3  two  and  one  half  times 

the  height  of  the  diagram.    A  filter  of  0.35  mm.  of  zircon  reduces  the  intensity  of  the  a 

line  from  62  to  21 .4;  while  reducing  the  j8 

line  from  39  to  2.2.  The  general  radiation 

to  the  left  is  still  quite  prominent.    An 

increase  in  filter  thickness  from  0.35  mm. 

to  0.58  mm.  (Curve  C)  reduces  it  but 

little  more  than  it  reduces  the  a  line,  so 

that  very  little  is  gained  by  additional 

filtering.     The    sudden  increase  in  ab-  $ 

sorption   of    the   zirconium    is   seen   at  fc 

\o  =  0.690  A.,  which  is  exactly  the  short  s 

wave-length   limit    of    its  K  series,   as 

extrapolated   from   M aimer's  values  of 

the  |8i  and  j82  lines  of   yttrium  and  the 

/Si  line  of  zirconium. 

The  effect  of  a  tungsten  filter  upon 

the  spectrum  of  tungsten  at  110,000  volts 

is  shown  in  Figs.  4  and  5.    Here   the 

critical  wave-length  of  the  filter  is  at  the 

short  wave-length  edge  of  the  whole  series,  so  that  all  the  lines  are  present.     A  filter 

of  ytterbium  would  eliminate  all  but  the  o:  doublet.  Fig.  4  gives  the  ionization  chamber 

1  The  absorption  of  the  Si  and  O  in  zircon  is  negligible  compared  to  that  of  the  zirconium,  so  that  crystal  zircon  is  as 
efficient  as  metallic  zirconium. 

2  The  ionization  chamber  contains  two  electrodes  of  equal  length.     The  second  electrode,  the  one  farther  from  the 
crystal,  was  connected  to  the  electrometer,  and  the  pressure  of  methyl  iodide  in  the  chamber  was  such  that  the  wave-lengths 
in  the  middle  of  the  range  investigated  suffered  50  per  cent  absorp^n  in  passing  through  the  first  half  of  the  chamber.    The 
electrometer  deflection  is  proportional  to  loe-pl  (l — e-pl),  where  /o  is  the  intensity  on  entering  the  chamber,  I  the  length  of 
either  electrode  and   it  the  coefficient  of  absorption  of  the  methyl  odide.     This  expression  has  a  very  flat  maximum  for 
c~id  =\,  so  that  for  a  considerable  range  on  either  side,  the  readings  are  proportional  to  /o. 


Fig.  4 


184 


measurements,  uncorrected,  of  the  tungsten  spectrum  at  110,000  volts,  as  reflected  by 
a  rock  salt  crystal.  The  upper  curve  is  the  unfiltered  spectrum,  the  lower  that  which 
has  passed  through  a  filter  of  0.15  mm.  of  metallic  tungsten.  The  K  lines  are  much 
more  prominent  in  the  filtered  than  in  the  unfiltered  spectrum,  but  the  general  radia- 
tion, especially  the  short  wave-length  end,  is  much  too  prominent,  showing  that  the 
voltage  is  too  high.  In  Fig.  o  the  effect  of  the  tungsten  filter  (above)  is  compared  with 
that  of  1  cm.  of  aluminium  (below),  in  order  to  show  more  clearly  the  selective  effect 
of  the  tungsten  filter.  The  wide  middle  portion  of  the  spectrum  is  unfiltered. 

THE  CRYSTALLINE  MATERIAL 

The  Bragg  method  of  X-ray  crystal  analysis  is  by  far  the  simplest  whenever  single 
crystals  of  sufficient  perfection  are  available.  If,  however,  perfect  order  of  crystalline 
arrangement  cannot  be  had,  the  next  simplest  condition  is  perfect  chaos,  that  is,  a  ran- 
dom grouping  of  small  crystals,  such  that  there  is  equi-partition  of  reflecting  opportun- 
ity among  all  the  crystal  planes.  This  has  two  disadvantages,  viz.,  that  the  oppor- 
tunity of  any  one  plane  to  reflect  is  very  small,  so  that  long  exposures  are  necessary; 
and  the  images  from  all  planes  appear  on  the  same  plate,  so  that  it  is  impossible,  with- 
out calculation,  to  tell  which  image  belongs  to  which  plane.  It  has  the  advantages,  on 
the  other  hand,  of  allowing  a  definite  numerical  calculation  of  the  position  and  intensity 
of  each  line,  and  of  being  free  from  uncertainties  due  to  imperfection  and  twinning  of 
crystals.  In  the  latter  respect  it  serves  as  a  valuable  check  on  the  direct  Bragg  method. 

The  crystalline  material  is,  wherever  possible,  procured  in  the  form  of  a  fine  powder 
of  0.01  cm.  diameter  or  less.  This  may  be  accomplished  by  filing,  crushing,  or  by  chemi- 
cal or  electro-chemical  precipitation,  or  by  distillation.  In  the  case  of  the  metals  like 
alkalies,  to  which  none  of  these  methods  can  be  applied,  satisfactory  results  have  been 
obtained  by  squirting  the  metal  through  a  die  in  the  form  of  a  very  fine  wire,  which  is 
packed,  with  random  folding,  into  a  small  glass  tube,  and  kept  in  continuous  rotation, 
with  frequent  vertical  displacements,  during  exposure. 

The  method  of  mounting  the  crystalline  substance  depends  on  the  wave-length  used  . 
If  tungsten  rays  (X  =  0.212)  are  used,  so  that  the  angles  of  reflection,  for  all  visible  lines, 
are  small,  it  is  most  convenient  to  press  the  powder  into  a  flat  sheet,  or  between  plane 
glass  plates,  and  place  this  sheet  at  right  angles  to  the  beam.  In  this  cas'e  the  correction 
for  the  difference  in  absorption  of  the  different  diffracted  rays  is  negligible.  If  a  molyb- 
denum tube  is  used,  on  the  other  hand,  diffracted  rays  can  be  observed  at  angles  up  to 
ISO  deg.  (cf.  Fig.  10),  so  that  the  substance  must  be  mounted  in  a  cylindrical  tube.  In 
this  case  also,  the  correction  for  absorption  is  unnecessary,  provided  the  diameter  of  the 
tube  is  properly  chosen  and  the  beam  of  rays  is  wide  enough  to  illuminate  the  whole 
tube. 

The  optimum  thickness  of  crystalline  material,  for  a  given  wave-length,  may  be 
calculated  approximately  as  follows: 

Let  k  represent  the  scattering  coefficient  and  ju  the  absorption  coefficient  of  the  sub- 
stance for  the  wave-length  used,  and  70  the  intensity  of  the  incident  rays.  The  intensity 
scattered  b  a  thin  laer  dx  at  a  distance  x  below  the  surface  will  be 


This  radiation  will  suffer  further  absorption  in  passing  through  a  thickness  t-x,  ap- 
proximately, where  /  is  the  thickness  of  the  sheet.  Hence  the  total  intensity  of  the 
scattered  radiation  that  emerges  will  be 


185 


This  will  be  a  maximum  when 

dR_         _itt         _j  _ 

or 

M' 

where  t  is  the  thickness  of  the  crystalline  sheet  in  centimeters  and  JJL  the  linear  ab- 
sorption coefficient. 

If  the  material  is  in  cylindrical  form,  the  optimum  diameter  is  slightly  greater  than 
the  above  value. 

EXPOSURE 

Very  long  exposures,  as  remarked  above, 'are  necessary  if  a  large  number  of  lines  is 
desired,  and  it  is  important  to  increase  the  speed  by  the  use  of  an  intensifying  screen, 
and  by  bringing  the  crystal  as  close  as  practicable  to  the  tube.  With  rays  as  absorbable 
as  those  from  a  molybdenum  tube,  it  is  necessary  to  use  films,  not  plates,  with  the  in- 
tensifying screen.  Under  reasonable  conditions,  an  exposure  of  ten  to  twenty  hours 
will  produce  a  general  blackening  of  the  plate  well  within  the  limit  of  normal  exposure. 
Since  a  greater  density  than  this  cannot  increase  the  contrast,  nothing  is  to  be  gained  by 
longer  exposure.  Further  detail  can  be  hoped  for  only  by  using  more  nearly  mono- 
chromatic rays,  screening  the  plate  more  perfectly  from  stray  and  secondary  rays  in  the 
room,  and  decreasing  the  ratio  of  amorphous  to  crystalline  material  in  the  specimen  un- 
der examination. 

ANALYSIS  OF  THE  PHOTOGRAPHS 

A.  Cubic  Crystals 

The  method  of  deducing  the  crystal  structure  from  the  experimental  data  is  very 
similar  to  that  used  by  the  Braggs,  with  this  difference:  In  the  Bragg  method  re- 
flections from  three  or  four  known  planes  are  observed,  and  a  structure  is  sought  which 
gives  the  spacings  and  intensities  observed  for  these  planes.  In  the  method  described 
above  a  single  photograph  is  taken,  containing  reflections  from  a  large  number  of  un- 
known planes,  and  a  structure  is  sought  whose  whole  pattern  of  planes,  arranged  in  the 
order  of  decreasing  spacing  and  omitting  none,  fits  the  observed  pattern.  In  both  cases 
the  method  is  one  of  trial  and  error,  namely,  to  try  one  arrangement  after  another, 
beginning  with  the  simplest,  until  one  is  found  which  fits. 

CALCULATION  OF  THEORETICAL  CRYSTAL  SPACINGS 

The  process  of  calculating  the  spacings  of  the  planes  in  any  assumed  crystal  structure 
is  as  follows :  The  positions  of  the  atoms  are  specified  by  their  co-ordinates  with  respect 
to  the  crystallographic  axes.  For  example,  a  centered  cubic  lattice  is  represented  by  a 

system  of  atoms  whose  co-ordinates  (x,  y,  z)  are  <          j,         IV        ,/  where  m,  n,  and  p 


assume  all  possible  integral  values,  and  the  unit  is  the  side  of  the  elementary  cube.  The 
distance  from  any  atom  xit  y\,  z\,  to  a  plane  whose  (Miller)  indices  are  h,  k,  I  is,  for 
rectangular  axes, 


Since  the  family  of  planes  parallel  to  h,  k,  I,  contains  all  the  atoms  in  the  crystal,  one  of 
these  planes  must  pass  through  the  atom  x\,  y\,  z\,  so  that  d  is  the  distance  from  the  plane 
h,  k,  I  to  a  plane  parallel  to  it  through  %\,  y\,  z\.  The  "spacing"  of  the  planes  h,  k,  I, 
which  is  the  smallest  value  of  d  that  repeats  itself,  is  found  by  substituting  different 


186 


TABLE  II 


Indices  of  Form 

Smallest  Distance 
Between  Planes 
d. 

SPACING  OF  PLANES  AND  SUBMULTIPLES  d/1t. 

n=l. 

2. 

3. 

100 
110 

111 

210 
310 
410 
320 
311 
411 
511 
711 
911 
322 
533 
733 
221 
331 
551 
553 
321 
531 
731 
751 
753 

5 

V'l 

i 

V2 

V3 

i 

V5 
1 

.50 

.354 
.577 
.224 
.158 
.121 
.139 
.302 
.118 
.192 
.140 
.110 
.121 
.152 
.122 
.167 
.115 
.140 
.130 
.134 
.168 
.130 
.115 
.110 

.25 
.177 
.289 
.112 
.079 
.061 
.070 
.151 
.059 
.096 
.070 
.055 
.016 
.076 
.061 
.084 
.058 
.070 
.065 
.067 
.084 
.065 
.058 
.055 

.167 
.118 
.192 
.075 
.053 
.040 
.046 
.101 
.039 
.064 
.047 
.037 
.040 
.051 
.041 
.056 
.038 
.047 
.043 
.045 
.056 
.043 
.038 
.037 

vio 

I 

V17 

2 

V13 
1 

VI  1 

5 

v'18 

1 

V727 
1 

VX51 
1 

V83 

i 

2 

X/17 
1 

V43 
1 

\/6~ 

i 

V9 
1 

Vl9 
1 

V51 
1 

V59 
i 

V14 
1 

V35 

1 

V59 
1 

V75 
1 

v'83 

187 


values  of  the  co-ordinates  m,  n,  p  for  xi,  y\,  :i  in  equation  (1),  and  observing  the  smallest 
value  and  periodicity  of  d.  For  example,  to  find  the  spacing  of  the  1  1  1  planes  of  the 
centered  cubic  lattice,  place  h,  k,  I  each  equal  to  1,  assume  for  m,  n  and  p  the  values  0, 
1  ,  2,  3,  etc.,  and  substitute  in  equation  (1).  All  the  atoms  in  the  group  m,  n,  p  are  found 

1        2       3 

to  lie  in  planes  at  distances  ~~r~,  "7=  ,  ~~F  ,  etc.,  from  k,  k,  I,  and  those  of  the  group 

v3   v3 


,  at  distances  ~~^=,  -^=,  ~~r=,  etc.      Since  both  groups  contain  the 
"V  o    'v  o     \  o 


same  number  of  atoms,  the  spacing  is  regular  and  is  equal  to  ~^  times  the  side  of  the 

elementary  cube.    If  the  structure  is  one  of  the  fourteen  regular  lattices,  as  the  centered 
cube,  all  parallel  planes  are  equally  spaced,  and  only  the  minimum  value  of  d  need  be  found. 

In  this  way  the  spacings  of  all  the  principal  planes,  that  is,  those  whose  indices  are 
small  numbers,  are  calculated  and  tabulated;  and  it  is  easy,  by  systematic  procedure, 
to  be  sure  that  no  plane  has  been  skipped  whose  spacing  is  within  the  limits  of  the  table. 

As  an  example,  the  calculation  of  the  spacings  of  a  face-centered  lattice  is  given  in 
full  below  (Table  II). 

The  co-ordinates  of  the  atoms  are 

m,  n,  p, 

m-\-lA.,  n  +^2,  p, 
m  +  ^2,  n, 
m,  n  +  1/ 

where  m,  n,  and  p  assume  all  possible  integral  values. 

The  first  column  gives  the  indices  of  the  form,  the  second  the  smallest  values  of  d 
for  that  form,  obtained  by  substituting  the  co-ordinates  of  the  atoms  in  equation  (1), 
and  the  third  the  same  value  of  d  expressed  as  a  fraction  of  the  lattice-constant,  together 
with  its  submultiples  d/2,  d/3,  d/4,  etc.,  corresponding  to  reflections  of  second,  third, 
etc.,  order.  The  unit  is  the  ''  lattice  constant,  "  the  side  of  the  elementary  face-centered 
cube.  The  table  contains  all  planes  having  values  of  d/n  greater  than  0.12.  These 
values  are  collected  in  Table  III,  arranged  in  order  of  decreasing  d/n. 

For  convenience  of  reference,  the  spacings,  i.e.,  the  distance  between  consecutive 
parallel  planes,  of  the  most  important  forms,  in  the  four  most  common  cubic  lattices  are 
tabulated  in  Table  III,  together  with  such  submultiples,  d/n,  of  these  spacings  as  come 
within  the  range  of  the  tables.  The  order  is  that  of  decreasing  d/n,  and  the  table  con- 
tains all  values  of  d/n  greater  than  0.12.1  The  table  also  contains  the  number  of  differ- 
ent sets  of  planes  in  each  of  the  given  forms.  For  example,  the  hexahedral  form  (100) 
consists  of  three  families  of  parallel  planes,  parallel  respectively  to  100,  010,  and  001. 

To  test  whether  any  new  crystal  belongs  to  one  of  the  lattices  represented  in  Table 
III,  it  is  only  necessary  to  calculate  the  values  of  d/n  from  the  lines  of  its  powder  photo- 
graph, tabulate  them  in  order,  and  compare  this  table  with  Table  III. 

The  unit  of  d/n  in  Table  III  is  the  "lattice  constant,"  i.e.,  the  side  of  the  elementary 
cube  whose  successive  translations  can  generate  the  whole  lattice.  To  find  the  spacing 
of  any  set  of  planes  in  a  crystal  having  one  of  these  lattices  it  is  only  necessary  to 
multiply  the  value  of  d  given  in  the  table  by  the  '  lattice  constant  "  of  the  given  crystal. 

The  first  three  lattices  in  the  table  are  the  regular  cubic  space  lattices,  in  which 
every  atom  is  equivalent  in  position  to  every  other.  In  all  other  possible  cubic  lattices 
the  atoms  must  be  divided  into  two  or  more  classes,  whose  positions  in  the  lattice  are  not 
equivalent. 

J  In  order  to  shorten  the  table  the  simple  cube  spacings,  which  are  much  more  numerous  than  the  others,  have  not 
been'tabulated  beyond  d/n  =.1766. 

188 


TABLE  III 


Indices  of 
Form 

Plane 
Families 
Belonging 
to  Form 

SPACING  OF  PLANES.  INCLUDING  SUBMULTIPLE  d/n 

Simple  Cube 

Centered  Cube 

Face-centered 
Cube 

Diamond 

100 

3 

1.00 

110 

6 

.707 

.707 

111 

4 

.577 

.577 

.577 

100 

3 

(n=2)  .500 

.500 

.500 

210 

12 

.447 

211 

12 

.   .408 

.408 

• 

110 

6 

(n=2)  .354 

(n=2)  .354 

.354 

.354 

/221 

/12 

.3333\ 

\  100 

\  3 

(n=3)  .3333  / 

310 

12 

.3160 

.3160 

311 

12 

.3014 

.301 

.301 

111 

4 

(w=2)  .2885 

.2885     (»=2)  .2885 

(n=2)  .2885 

320 

12 

.2774 

321 

24 

.2672 

.2672 

100 

3 

(n=4)  .2500 

(n=2)  .2500     (n=2)  .2500 

.2500 

/410 

/12 

.2423  \ 

•       J 

1  322 

112 

.2423  j 

J411 

J12 

.2358  1 

.2358  \ 

\  110 

\  6 

(n=3)  .2358  j 

(n=3)  .2358  j 

331 

12 

.2292 

.2292 

.2292 

210 

12 

(n=2)  .2234 

.2234 

.2234 

421 

24 

.2180 

332 

12 

.2132 

.2132 

211 

12 

(n=2)  .2040     (n=2)  .2040 

.2040 

.2040 

/  430 

/12 

.200  \ 

100 

\  3 

(»=5)  .200  J 

/431 

J24 

.1960  1 

.1960  \ 

1  510 

1  12 

.1960  / 

.  1960  J 

/oil 

112 

.1923  1 

.1923  1 

.19231 

1111 

\  4 

(n=3)  .1923  j 

(n=3)  .1923  / 

(n=3)  .1923  / 

J  520 

/12 

.1856  \ 

\432 

\24 

.1856  j 

521 

24 

.1826 

.1826 

110 

6 

(n=4)  .1766 

(n=4)  .1766     (n=2)  .1766 

(n=2)  .1766 

/530 

12 

.1714  1 

1433 

12 

.1714  J 

531 

24 

.169 

.169 

f  100 

3 

(n=3)  .167  \    (n=3)  .167  \ 

]  221 

12 

.167  J          .167  J 

J611 

12 

.1621 

1  532 

24 

.1621 

310 

12 

(w=2)  .1580           .1580 

.1580 

533 

12 

.1525 

.1525 

311 

12 

.1507     (n=2)  .1507 

(n=2)  .1507 

631 

24 

.1474 

111 

4 

(n=2)  .1442     (w=4)  .1442 

(n=4)  .1442 

f  110 

6 

(n=5)  .1414  1 

\  710 

12 

.1414  f 

!  543 

24 

.1414  J 

J711 

12 

.1400  1 

.14001 

1551 

12 

.1400  J 

.1400  f 

320 

12 

.1387 

.1387 

f  211 

12 

(n=3)  .1360  ] 

\  552 

12 

.1360  [• 

1721 

24 

.1360  j 

321 

24 

(w=2)  .1336 

.1336 

.1336 

730 

12 

.1312 

/553 

12 

.1301  1 

.1301  \ 

1  731 

24 

.1301  f 

.1301  / 

j  732 

24 

.1270\ 

651 

24 

.1270  / 

100 

3 

(w=4)  .1250 

(«=4)  .1250 

(n=2)  .1250 

l  741 

24 

.1230  1 

<  811 

12 

.1230  [ 

I  554 

12 

.1230  J 

733 

12 

.1222 

.1222 

/410 

12 

.12121 

.12121 

1  322 

12 

.1212  / 

.1212  / 

189 


The  first  lattice  is  the  simple  cubic  lattice,  the  unit  of  whose  structure  is  a  cube 
with  atoms  at  each  corner.  The  positions  of  the  atoms  are  specified  by  giving  to  each 
of  the  co-ordinates  m,  n,  p  all  possible  integral  values  within  the  limits  of  the  size  of 
the  crystal.  Each  atom  in  the  lattice  has  as  nearest  neighbors  six  symmetrically  placed 
atoms,  which  form  an  octahedron  about  it.  This  arrangement  of  atoms  is  exemplified 
by  rock  salt,  except  that  in  rock  salt  the  atoms  are  alternately  sodium  and  chlorine. 
No  elementary  substance  with  simple  cubic  structure  has  yet  been  found. 

The  second  lattice  is  a  centered  cubic  lattice,  whose  unit  is  a  cube  with  an  atom  in 
each  corner,  and  one  at  the  center  of  the  cube.  It  may  be  formed  by  superimposing  two 
simple  cubic  lattices  in  such  manner  that  the  atoms  of  the  one  are  at  the  centers  of  the 
cubes  of  the  other.  The  co-ordinates  of  the  atoms  are  therefore  given  by 

^  m,  n,  p, 


where  m,  n,  and  p  have  all  possible  integral  values.  Each  atom  in  this  lattice  has  eight 
equidistant  nearest  neighbors,  which  form  a  cube  about  it.  Examples  of  this  structure 
are  iron  and  sodium. 

The  third  lattice  is  the  face-centered  cubic  lattice.  Its  unit  of  structure  is  a  cube  with 
an  atom  at  each  corner  and  one  in  the  center  of  each  face.  It  may  be  formed  by  the 
superposition  of  four  simple  cubic  lattices  with  construction  points1  o,  o,  o;  J^,  J/2,  o; 
y^i  °>  y^\  o,  %,  ^2,  respectively.  The  co-ordinates  of  the  atoms  are  therefore 

m,  n,  p, 

p, 


where  m,  n,  and  p  have  all  possible  integral  values.  Each  atom  in  this  lattice  is  sur- 
rounded by  twelve  equidistant  atoms  which  form  a  regular  dodecahedron  about  it.  Ex- 
amples of  this  structure  are  aluminium,  copper,  silver,  gold  and  lead. 

The  fourth  lattice  is  known  as  the  diamond  type  of  lattice,  and  is  exemplified  by 
diamond  and  silicon.  It  may  be  formed  by  the  superposition  of  two  face-centered 
lattices,  with  construction  points  o,  o,  o,  and  34,  M.  34  respectively.  The  co-ordinates 
of  the  atoms  are  therefore 

m,  n,  p, 

n-\-}/2,  p, 


,  p-\-l/2, 


where  m,  n,  and  p  have  all  possible  integral  values. 

Each  atom  in  this  lattice  is  surrounded  by  four  equidistant  atoms,  which  form 
a  tetrahedron  about  it.  The  tetrahedra,  however,  are  not  all  similarly  situated,  half  of 
the  atoms  being  surrounded  by  positive  tetrahedra,  and  the  other  half  by  negative 
tetrahedra.  In  this  lattice  successive  parallel  planes  are  not  all  equidistant.  In  those 
forms  whose  indices  are  all  odd  numbers,  as  (751),  (533),  the  planes  are  arranged  in 
regularly  spaced  pairs,  the  distance  between  members  of  a  pair  being  one  fourth  the  dis- 
tance between  consecutive  pairs.  In  all  other  forms,  that  is,  those  whose  indices  are 
not  all  odd,  the  spacing  is  regular. 

1  The  term  "construction  point  "  is  used  to  denote  the  position  of  some  definite  point,  which  may  be  looked  upon  as  the 
starting  point  of  each  lattice,  with  respect  to  the  co-ordinate  axes. 

190 


B.  Crystals  Other  Than  Cubic 

In  the  case  of  crystals  belonging  to  systems  other  than  the  cubic,  the  procedure  is 
not  so  simple.  It  is  necessary  to  make  a  separate  calculation,  not  only  for  every  kind  of  ' 
atomic  grouping,  but  for  every  different  ratio  of  the  axes  or  angle  between  axes.  When 
these  axes  are  not  known  from  crystallographic  data,  as  in  the  case  of  graphite,  for  ex- 
ample, a  great  many  trials  have  to  be  made  before  the  correct  one  is  found.  Also,  in  the 
case  of  oblique  axes  the  formula  for  the  distance  between  planes  is  less  simple.  How- 
ever, when  the  crystallographic  data  is  reliable  the  process  is  not  difficult.  A  few  ex- 
amples will  be  given  below  for  illustration  and  reference. 

The  general  formula  for  the  distance  from  a  plane  h,  k,  I  (Miller  indices)  to  a  parallel 
plane  through  the  point  x\,  y\,  z\,  referred  to  any  system  of  axes  X,  Y,  Z,  having  angles 
X,  ju,  v  between  the  axes  YZ,  XZ,  and  XY  respectively,  is1 


a  =' 


< 

h 

h  cosy  cos/* 
k  1   cosX 
/  cosX   1 

+  k 

1   h  cosju 

COSl>  k  COSX 
COSM  /   1 

+' 

1   cose  h 

COSy    1    k 

cos  p.  cosX  / 

i 

1     COS?  COS/X 

cosy   1   cosX 

COS/i   COSX    1 

For  the  three  rectanglar  systems,  the  cubic,  tetragonal  and  orthorhombic,  X,  /*,  and 
v  are  each  90  deg.  and  equation  (2)  reduces  to  equation  (1).  For  the  tetragonal  and 
orthorhombic  systems,  however,  and  in  all  the  other  systems  except  the  cubic  and  trig- 
onal, the  co-ordinates  x\,  y\,  TI,  and  the  indices  h,  k,  and  /  are  not  all  measured  in  the 
same  units.  The  products  hxi,  ky\,  lz\,  of  the  numerator  are  of  zero  dimensions,  but  the 
values  of  h,  k,  and  /  in  the  denominator  contain  the  units,  and  must  be  replaced  by 
h/a,  k/b,  L/C,  where  a,  b,  and  c  are  the  unit  axes  of  the  crystal  in  theA,  Y,  and  Z  direc- 
tions respectively.  This  gives: 

For  the  tetragonal  system 

w-  -+-  kyl  H-fei-1 


(4) 


d  =  - 

Vh*  +  k2  +  (//c)2 

where  c  is  the  axial  ratio  of  the  crystal;  and  for  the  orthorhombic  svstem 


d  = 


+  kyi  +  /n  —  1 


where  a  and  c  are  the  lengths  of  the  shorter  lateral  axis  and  vertical  axis  respectively. 
For  the  hexagonal  system,  if  two  of  the  horizontal  axes,  120  deg.  apart,  are  taken  as 
A"  and  Y,  and  the  vertical  axis  as  Z,  X  and  /*  are  each  90  deg.  and  v  120  deg.,  and  equation 
(2)  reduces  to 


i 

~  V4/3(fe8+/tA?+fes)  +  (//c)*' 
where  c  is  the  "axial  ratio"  for  the  particular  crystal  species. 

1  This  formula  is  easily  obtained  from  the  fundamental  equation 

d  =xi  cos  a-\-yi  cos  ft-\-zi  cos  y  —  p, 

by  substituting  for  cos  a  cos  ft.  cos  7,  and  v  their  values  in  terms  of  h,  k,  I,  X,  n,  and  v  given  by  the  equations: 

cos  a  =/'-(-  m  cos  K+«  cos  M  =hp 
cos  0  =1'  cos  v-^-m-\-n  cos  X  =kp 
cos  7  =/'  cos  /i-)-»i  cos  X-|-n  =lp 

I'  cos  a-\-  TO  cos  0-|-  n  cos  7  =  1 

where  cos  a.  cos  ft,  cos  7  are  the  direction  cosines,  and  /',  m,  n  the  direction  ratios  of  the  perpendicular  />  from  the  origin  to 
the  plane  hkl. 


191 


For  the  trigonal  system,  in  which  X  =  /z=  v,  equation  2  reduces  to 


-3  cos2  X) 


(cos2X  — cosX) 
For  the  monoclinic  system  X  and  v  are  each  90  deg.  and  equation  (2)  becomes 

d  = 


,0X0' 

—  2  —  cos  /J 

ac 

_|_  fc2 

\                   sin2 

n 

where  a  and  c  refer  to  the  lengths  of  the  clinodiagonal  and  vertical  axes  respectively, 
the  orthodiagonal  axis  b  being  taken  as  unity. 

Finally,  for  the  triclinic  system,  for  which  the  general  equation  (2)  must  be  used,  it 
should  be  noted  that  in  order  to  use  the  equation  for  numerical  calculation,  the  quanti- 
ties h,  k,  and  /  in  the  denominator  should  be  divided  by  the  corresponding  axial  lengths 
a,  b,  c. 

Standard  tables  of  calculated  spacings.  like  Table  III  above,  cannot  be  given  for 
crystal  systems  other  than  the  isometric,  since  the  axial  ratios  and  angles  are  different 
for  each  crystal.  By  way  of  example,  however,  the  spacings  of  three  hexagonal  lattices 
having  the  axial  ratio  1.624,  which  is  the  accepted  value  for  magnesium,  are  given  in 
Table  IV.  The  first  is  a  simple  lattice  of  triangular  prisms,  the  length  of  whose  side  is 
taken  as  unity  and  whose  height  is  therefore  1.624.  It  is  one  of  the  regular  space  lat- 
tices. The  positions  of  the  atoms  in  this  lattice  are  given,  in  hexagonal  co-ordinates  (see 
equation  (5)  above)  by  (x,  y,  z  =  )  m,  n,  pc  where  each  of  the  co-ordinates  m,  n,  and  p 
assumes  all  possible  integral  values,  and  c  is  the  axial  ratio.  The  second  lattice  in  Table 
IV  is  composed  of  two  of  the  above  triangular  lattices  intermeshed  in  such  a  way  that 
the  atoms  of  the  first  are  in  the  centers  of  the  prisms  of  the  second  and  vice  versa.  It 
differs  but  very  little  from  the  so-called  hexagonal  close-packing,  which  is  one  of  the  two 
alternative  arrangements  which  the  atoms  would  assume  if  they  were  hard  spheres  and 
were  forced  by  pressure  into  the  closest  possible  packing.  The  positions  of  the  atoms 

are  given  bv  {  where  the  co-ordinates  refer  to  hexagonal  axes, 


and  m,  n,  p,  have  all  possible  integral  values.  The  third  lattice  in  Table  IV  is  composed 
of  three  of  the  above  simple  triangular  lattices,  the  atoms  of  the  second  and  third  being 
directly  above  the  centers  of  the  alternate  triangles  of  the  first  lattice,  at  distances  of  % 
and  %  respectively  of  the  height  of  the  prism.  It  is  the  regular  rhombohedral  lattice. 
The  positions  of  the  atoms  in  this  lattice  may  be  most  simply  specified  with  reference  to 
trigonal  axes,  but  for  convenience  of  comparison  with  the  first  two  lattices,  they  are 
given  in  terms  of  hexagonal  axes.  The  hexagonal  co-ordinates  are 


+  %,»+  ^,  (p 

where  m,  n,  and  p  have  all  possible  integral  values,  and  c  is  the  axial  ratio  1.624. 

The  first  column  in  Table  IV  gives  the  indices1  of  the  form,  the  second,  fourth  and 
sixth  the  number  of  different  families  of  planes  belonging  to  the  form,  and  the  third, 
fifth  and  seventh  the  spacing  of  these  planes  in  the  respective  lattices,  found  by  sub- 
stituting the  co-ordinates  of  the  atoms  in  equation  (5)  .  The  unit  is  the  side  of  the  ele- 
mentary triangular  prism.  The  eighth  column  gives,  for  comparison,  the  indices  of  the 

192 


TABLE  IV 


Indices 
of 
Form 

SIMPLE   TRIANGULAR 
LATTICE 

CLOSE  PACKED   LATTICE 

RHOMBOHEDRAL   LATTICE 

Number 
of  Co-                 Spacing  of 
operating                 Planes 
Planes 

Number 
of  Co-                 Spacing  of 
operating                  Planes 
Planes 

Number                                                    Tr'e        1 
of  Co-                 Spacing  of 
operating                 Planes                    ?  p 
Planes 

0001 

1                           1.624 

1010 

3                            .866 

3                            .866 

0001 

1              (n=2)  .812 

1                            .812 

1011 

6 

.764 

6                            .764 

3                            .764 

100 

1012 

6 

.592 

6                            .592 

3 

.592 

110 

0001 

1 

(n=3)   .541 

1 

.541 

111 

1120 

3 

.500 

3                            .500 

3 

.500 

110 

1121 

6 

.477 

1013 

6 

.458 

6                            .458 

1010 

3 

(n=2)   .433 

3              (n=2)   .433 

1122 

6 

.426 

6                            .426 

2021 

6 

.418 

6                            .418 

3 

.418 

111 

0001 

1 

(n=4)   .406 

1              (n=2)   .406 

1011 

6 

(n=2)   .382 

6              (n=2)   .382 

3              (n=2)   .382            100 

10l4 

6 

.368 

6                            .368              3                            .368            211 

1123 

6 

.367 

s* 

.367 

210 

2023 

6 

.338 

6                            .338 

2130 

6 

.327 

6                            .327 

0001 

1 

(n=5)   .325 

2131 

12 

.321 

12                            .321 

6 

.321 

210 

1124 

6 

.315 

6                            .315 

/  1015 
\2132 

6 
12 

.304 
.304 

6                            .304 
12                            .304 

O 

6 

.304 
.304 

221 
211 

1012 

6 

(n=2)   .296 

6              (w=2)   .296              3              (n=2)   .296 

110 

1010 

3               (n=3)   .289 

3               (n=3)   .289               3                             .289 

211 

planes  of  the  rhombohedral  lattice  in  trigonal  (Miller)  co-ordinates.  The  atoms  are  in 
this  case  referred  to  three  equal  axes,  making  equal  angles  of  78.4  deg.  with  each  other, 
and  their  co-ordinates  are  m,  n,  p,  where  each  of  these  numbers  has  all  possible  integral 
values. 

EXAMPLES 

As  examples  of  the  application  of  the  method  of  analysis  described  above,  the 
analysis  of  ten  elementary  crystalline  substances  is  given  below.  Three  of  these 
analyses  are  incomplete,  but  are  of  such  importance  as  to  warrant  their  inclusion.  Four 
others  have  already  been  briefly  described  elsewhere,  and  are  given  here  in  more  detail. 
The  last,  diamond,  which  has  been  completely  analyzed  by  the  Braggs,  is  added  as  a 
check  upon  the  method,  and  as  an  example  of  the  immense  amount  of  information 
which  can  be  obtained  from  a  single  photograph. 

The  experimental  data  is  collected  in  Tables  V  to  XIV.  The  first  column  in  each 
table  gives  the  estimated  intensity  of  each  line.  The  estimate  is  necessarily  very  rough, 
but  photometric  measurements  have  little  value  unless  care  is  taken  to  make  control  ex- 
posures to  determine  the  characteristic  curve  of  the  plate  under  the  actual  conditions  of 
exposure  and  development.  In  the  photographs  here  described,  this  was  not  done. 
The  second  column  gives  the  distance,  x,  of  each  line  on  the  photograph  from  the  cen- 
tral undeviated  image  of  the  slit.  The  third  column  gives  the  angular  deviation  20,  of 
the  ray  that  produced  the  line,  calculated  from  x  and  the  distance  between  crystalline 
material  and  photographic  plate.  The  fourth  and  fifth  columns  give  the  experimental 


193 


and  theoretical  values  of  d/n,  where  d  is  the  distance  in  Angstroms  between  consecutive 
planes,  and  n  the  order  of  reflection.  The  experimental  values  of  d/n  are  calculated 
from  the  angular  deviation  29  by  means  of  the  equation  n\  =  2d  sin  6.  The  theoretical 
values  are  obtained  by  multiplying  the  values  in  Table  III  and  IV  by  the  lattice  con- 
stants of  the  respective  crystals.  The  sixth  column  gives  the  indices  of  the  forms  to 
which  the  reflecting  planes  belong,  and  the  last  column  the  number  of  families  of  planes 
belonging  to  the  given  form  and  having  the  same  spacing,  so  that  their  reflections  are 
superimposed.  The  number  of  these  co-operating  planes  is  a  measure  of  the  intensity 
of  the  line  to  be  expected  if  the  atoms  are  symmetrical  and  equally  distributed  in  suc- 
cessive planes. 

IRON 

The  iron  investigated  was  obtained  from  two  sources,  viz.,  fine  filings  of  pure 
electrolytic  iron,  and  fine  iron  powder  obtained  by  the  reduction  of  Fe2O3  in  hydrogen. 
The  fillings  were  mounted  in  a  thin-walled  glass  tube  2  mm.  in  diameter,  which  was 
kept  in  rotation  during  the  exposure.  The  reduced  oxide  was  pressed  into  a  sheet  2 
mm.  thick,  which  was  mounted  firmly  at  right  angles  to  the  beam  of  X-rays.  Both 
specimens  gave  the  same  lines. 

A  fine-focus  Coolidge  X-ray  tube  with  tungsten  target  was  used  for  all  the  iron 
photographs.  It  was  operated  by  the  constant  potential  equipment  which  has  been  in 
use  for  two  years  in  the  Research  Laboratory,1  at  110,000  volts  and  1  milliampere. 

Fig.  6  shows  one  of  the  photographs  of  the  iron  powder  (reduced  oxide).  For  all 
lines  beyond  the  first  three,  the  a  doublet  is  resolved  into  two  very  narrow,  sharp  lines. 
The  /3  line  of  the  K  radiation  is  visible  on  the  plate  for  some  of  the  stronger  reflections, 
but  is  easily  distinguishable  from  the  double  a  line.  In  this  exposure  both  slits  were 
very  narrow,  about  0.2  mm.  wide.  The  distance  from  X-ray  tube  to  first  slit  was  20 
cm.,  from  first  to  second  slit  15  cm.,  and  from  crystal  to  photographic  plate  18.15  cm. 
Seed  X-ray  plate  was  used,  with  calcium  tungstate  intensifying  screen.  The  exposure 
was  20  hours. 

The  lines  in  this  photograph  are  tabulated  in  Table  V,  together  with  the  calculated 
spacings,  as  described  above.  The  observed  spacings  (column  4)  agree  with  the 
theoretical  spacings  for  a  centered  cube  of  side  2.86  A.  (column  5)  within  the  limit  of  ac- 


TABLE  V 

Iron 

Intensity 

Distance  of 
Line  from 

Angular 
Deviation  of 

SPACING  OF  PLANES  IN 

Number  of 

of  Line 

Center 

Line  28 

ANGSTROMS 

Indices  of  Form 

Co-operating 

Estimated 

Cm. 

Degree 

Experimental 

Theoretical 

j*anes 

1.00 

1.87 

5.90 

2.05 

2.06 

110 

6      ' 

.46 

2.67 

8.40 

1.43 

1.43 

100 

3 

.54 

3.40 

10.30 

1.16 

1.16 

211 

12 

.24 

3.85 

11.96 

1.005 

1.01                   110  (n  =2) 

3 

.18 

4.32 

13.40 

.910 

.905                   310 

12 

.16 

4.75 

14.67 

.823 

.826 

111 

4 

.22 

5.17 

15.90 

.757 

.765 

321 

24 

.715 

100  (n=2) 

3 

.12 

5.92 

18.06                 .665 

.675 

/  110  (n=3) 
\411 

18 
12 

.03 

6.27 

19.06 

.633 

.638 

210 

12 

.02 

6.62 

20.06 

.600 

.610 

332 

12 

.02 

7.00 

21.10 

.572 

.584 

211  (w=2) 

(  M  n 

.10 

7.25 

21.78 

.555 

.560 

J    O1U 

\431 

36 

.02 

7.95 

23.16 

.522 

.522 

521 

24 

1  Phys.  Rev.,  7,  405,  1916.    For  a  fuller  description  see  G.  E.  Review,  19,  173,  March,  1916. 

194 


curacy  of  measurement  of  the  lines.  The  intensities  also  vary  in  the  manner  to  be  ex- 
pected, except  that  the  second  order  110  line  is  too  intense  and  the  second  order  100  too 
weak.  The  bearing  of  this  fact  on  the  question  of  the  arrangement  of  electrons  in  the 
iron  atom  has  been  discussed  elsewhere.1 

A  centered  cubic  lattice  should  have  two  atoms  associated  with  each  elementary 
cube.  By  equating  the  mass  of  the  n  atoms  in  an  elementary  cube  to  the  mass  of  the 
cube,  i.  e.,  its  volume  X  density  of  the  metal,  we  obtain 

pd*     7.86X2.863X10~24_0 
~  M  ~55.4Xl.663X10-24~ 

As  a  check  upon  this  analysis,  photographs  were  taken  of  single  crystals  of  silicon 
steel,  containing  about  3.5  per  cent  silicon,  which  were  mounted  on  the  spectrometer 
table  and  rotated  about  definite  axes.  Two  of  these  photographs  are  reproduced  in  Fig. 
7.  The  first,  Fig.  7a,  is  the  photograph  of  a  thin  crystal  about  5  mm.  square,  cut 
parallel  to  100.  It  was  mounted  12  cm.  from  the  photographic  plate,  with  its  100  face 
normal  to  a  beam  of  tungsten  rays,  and  rotated  slowly,  about  an  axis  perpendicular  to 
001,  for  a  few  degrees  on  each  side  of  the  center.  It  shows,  in  the  horizontal  plane,  the 
spectrum  of  the  tungsten  target  reflected  from  010,  and  at  45  deg.  and  135  deg.  the  same 
reflected  from  Oil  and  01 1  respectively.  The  two  K  lines  in  tungsten,  the  unresolved  a 
doublet  and  the  /3  line,  show  plainly  in  each  of  these  spectra,  and  the  distance  between 
the  a  doublets  of  the  right  and  left  spectra,  viz.,  3.56  cm.  for  010,  and  2.50  cm.  for  Oil, 
give  for  the  spacings  of  these  planes : 

c?oio=1.43  A., 
Jon  =  2.04  A. 

The  second  photograph,  Fig.  7b,  shows  the  result  of  rotating  a  thin  crystal  cut 
parallel  to  1 1 1 ,  and  mounted  normal  to  the  rays,  about  an  axis  perpendicular  to  1 10,  for 
a  few  degrees  on  each  side.  It  shows,  in  the  horizontal  plane^the  reflection  from  112 
and  at  30,  60  and  71  deg.  to  the  horizontal  the  reflections  from  101,  211,  321,  with  cor- 
responding reflection  from  231,  121  and  Oil  at  angles  of  109,  120  and  150  deg.  respec- 
tively to  the  horizontal.  The  planes  101  and  Oil  show  both  first  and  second  order 
spectra.  The  distances  between  a  lines  of  these  spectra  agree  excellently  for  planes  be- 
longing to  the  same  form,  and  give  for  the  spacing  of  the  planes  in  the  three  forms  rep- 
resented : 

cU=1.15A., 
fa -2.02  A., 
d32i  =  0.75  A. 

The  agreement  of  these  angles  and  spacings  with  the  theoretical  values  for  a  cen- 
tered cubic  lattice  indicates  that  the  position  of  the  atoms  in  iron  is  not  greatly  affected 
by  the  presence  of  3^  Per  cent  Si. 

A  series  of  photographs  of  one  of  these  crystals  at  liquid  air  temperature,  room  tem- 
perature, and  1000  deg.  C.  respectively  showed  no  observable  change,  even  in  intensi- 
ties. It  is  necessary  to  photograph  more  forms,  however,  before  definite  conclusions 
can  be  drawn  regarding  the  relation  of  a  to  /3  iron.  Several  photographs  of  iron  powder 
at  different  temperatures  between  700  deg.  C.  and  900  deg.  C.  were  spoiled,  either  by 
chemical  fog  due  to  the  heating  of  the  photographic  plate,  or  by  the  growth  of  the 
crystals  during  exposure,  thus  giving  only  a  few  large  spots  on  the  photograph. 

SILICON 

Small  crystals  of  metallic  silicon  were  crushed  in  a  mortar  and  sifted  through  a 
gauze  of  200  meshes  to  the  inch.  The  fine  powder  was  mounted  in  a  very  thin- walled 

i  Phys.  Rev.,  9,  84.  1917. 

195 


tube  of  lime  glass,  and  kept  in  continuous  rotation  during  a  four-hour  exposure  to  rays 
from  a  molybdenum  target,  running  at  32  kv.  constant  potential  and  8  milliamperes. 
A  filter  of  zircon  powder  .037  cm.  thick  reduced  the  spectrum  essentially  to  a  single  line, 
the  unresolved  a  doublet,  X  =  .712,  of  molybdenum,  as  shown  in  Fig.  3.  The  crystal  was 
15  cm.  from  the  X-ray  target  and  11.3  cm.  from  the  photographic  film,  which  was  bent 
in"  the  arc  of  a  circle,  with  the  crystal  at  the  center.  Both  slits  were  quite  wide,  about 
1  mm.,  and  about  5  cm.  apart.  Eastman  X-ray  film  was  used,  with  calcium  tungstate 
intensifying  screen. 

The  photograph  obtained  is  reproduced  in  Fig.  8,  and  the  measurements  are  given 
in  Table  VI.  The  spacings  tabulated  as  "theoretical "  are  those  of  a  lattice  of  the  dia- 
mond type,  i.e.,  two  intermeshed  face-centered  lattices,  each  of  side  5.43  A.,  one  lattice 
being  displaced,  with  reference  to  the  other,  along  the  cube  diagonal  a  distance  one 
fourth  the  length  of  the  diagonal.  The  agreement  is  perfect.  The  estimates  of  inten- 
sity are  not  accurate  enough  to  warrant  discussion. 

The  number  of  atoms  associated  with  each  unit  cube  is 


pds     2.34X5.433 


M     28.1X1.663 
which  is  the  correct  number  for  this  type  of  lattice. 


=  8.00, 


TABLE  VI 

Silicon 


Intensity 
of  Line 

Distance  of 
Line  from 
Center 

Angular 
Deviation  of 
Line  2  0 

SPACING  OF  PLANES  IN 
ANGSTROMS 

Indices  of  Form 

Number  of 
Coo  Derating 
Planes 

Estimated 

Cm. 

Degrees 

Experimental 

Theoretical 

1.00 

2.58 

13.56 

3.13- 

3.14 

111 

4 

.80 

4.21 

23.0 

1.93 

1.93 

110 

6 

.75 

4.97 

26.16 

1.64 

1.64 

311 

12 

0 

1.57 

111  (n=2) 

4 

.25 

6.00 

31.58 

1.36 

1.356 

100 

3 

.45 

6.54 

34.42 

1.25 

1.25 

331 

12 

.50 

7.39 

38.92 

1.11 

1.11 

211 

12 

.40 

7.84 

40.78 

1.05 

1.04 

/511 
\  111  (n=3) 

16 
16 

.20 

8.59 

45.20 

.96 

.96 

110  (n=2) 

6 

.30 

8.98 

47.22 

.92 

.92 

531 

24 

.25 

9.66 

50.44 

.86 

.86 

310 

12 

.10 

10.01 

52.44 

.83 

.83 

533 

12 

0 

.82 

311  (n=2) 

12 

.05 

10.62 

55.70 

.79 

.79 

111  (n=4) 

4 

.10 

11.00 

59.76 

.76 

.76 

/711 
1661 

24 

.20 

11.58 

60.36 

.73 

.73 

321 

24 

.15 

11.90 

62.56 

.71 

.71 

/731 
\553 

36 

0 

.68 

100  (n=2) 

3 

0 

.66 

733 

12 

.05 

13.33 

70.0 

.64 

.64 

411 

12 

.05 

13.60 

71.44 

.63 

.63 

/751 
\  111  (n=5) 

28 

ALUMINIUM 

Fine  filings  of  pure  sheet  aluminium  were  mounted  in  exactly  the  same  manner  as 
silicon,  and  exposed  for  3  hours  to  molybdenum  rays,  produced  at  40,000  volts,  9 
milliamperes,  and  filtered  by  .037  cm.  of  zircon  powder.  The.  photograph  obtained  is 
shown  in  Fig.  9,  and  the  measurements  are  given  in  Table  VII. 


196 


TABLE  VII 
Aluminium 


Intensity  of     j     Dg«£* 
Lme                    Centre 

Angular 
Deviation  of 
Line  2  8 

SPACING  OF  PLANES  IN 
ANGSTROMS 

Indices  of  Form 

Cooperating 
Planes 

Estimated 

Cm. 

Degrees 

Experimental 

Theoretical 

1.00 

3.45 

17.80 

2.33 

2.33 

111 

4 

.60 

3.99 

20.60 

2.025 

2.025 

100 

3 

.50 

5.67 

29.26 

1.43 

1.43 

110 

6 

.60 

6.68 

34.5 

1.21 

1.22 

311 

12 

.20 

6.95 

35.8 

1.17 

1.17 

111  (w=2) 

4 

.05 

8.09 

41.8 

1.01 

1.01 

100  (n=2) 

3 

.25 

8.86 

45.6 

.93 

.93 

331 

12 

.25 

9.11 

47.0 

.90 

.90                   210 

12 

.10 

10.05 

51.8 

.82 

.83                   211 

12 

.15 

10.66 

55.0 

.78 

.78                /511 

I  111  (n=3) 

16 

.02 

11.75 

60.6 

.71  ' 

.72                   110  (n  =2) 

6 

.04 

12.30 

63.4 

.68 

.68                   531 

24 

TABLE  VIII 

Magnesium 


Intensity  of 
Line 

Distance  of 
Line  from 
Center 

Angle  of 
Reflection 

SPACING  OF  PLANES  IN 
ANGSTROMS 

Indices  of  Form 

Number  of 
Cooperating 
Planes 

Estimated 

Cm. 

Degrees 

Experimental 

Theoretical 

.40 

2.92 

14.80 

2.75 

2.75 

1010 

3 

2.59 

0001 

1 

1.00 

3.30 

16.66 

2.44 

2.44 

1011 

6 

.30 

4.23 

21.48 

1.91 

1.90 

1012 

6 

.40 

5.03 

25.50 

1.61 

1.60 

1120 

3 

.35 

5.50 

27.8 

1.48 

1.48 

1013 

6 

1.38 

1010  (2) 

3 

.35 

5.95 

30.00 

1.36 

1.36 

1132 

6 

.12 

6.05 

30.6 

1.34 

1.34 

2021 

6 

1.30 

0001  (2) 

1 

.06 

6.65 

33.6 

1.23 

1.23 

1011  (2) 

6 

.02 

6.9 

34.8 

1.18 

1.18 

1014 

6 

.10 

7.52 

38.0 

1.09 

1.08 

2023 

6 

.18 

7.96 

40.2 

1.04 

1.05 

2130 

6 

.02 

8.1 

41.0 

1.02 

1.03 

2131 

12 

1.01 

1124 

6 

.12 

8.41 

42.6 

.98 

.97 

J  2132 
1  1015 

18 

.94 

1012  (2) 

6 

.01 

8.83               44.8 

.93 

.92 

1010  (3) 

3 

.10 

9.15 

46.2 

.90 

.89 

2133 

12 

.06 

9.48 

48.0 

.87 

.87 

J3032 
\  0001  (3) 

7 

.02 

9.90 

50.2 

.83 

.83 

/  1016 
1  2025 

12 

.82 

/  1011  (3) 
\2134 

18 

.80 

1120  (2) 

3 

.06 

10.95 

55.4 

.77 

.77 

3140 

6 

The  "theoretical"  spacings  in  Table  VII  are  those  of  a  face-centered  cubic  lattice. 
Their  agreement  with  the  "experimental "  values  obtained  from  the  lines  on  the  photo- 
graph is  satisfactory. 


197 


The  number  of  atoms  per  unit  elementary  cube  is 

p<f_2.70X(4.05)3 
~M~  2.69X1. 663  " 
This  is  the  correct  number  for  a  face-centered  lattice. 

The  unit  of  structure  of  the  aluminium  crystal  is,  therefore,  a  face-centered  cube,  of 
side  4.05  A.,  with  one  atom  of  aluminium  at  each  corner  and  one  at  the  center  of  each 
face. 

MAGNESIUM 

The  magnesium  used  in  these  experiments  was  the  commercial  electrolytic  product 
made  in  the  research  laboratory.  Several  photographs  were  taken,  some  with  fine  fil- 
ings from  cast  rods  of  this  metal,  and  some  with  filings  from  large  crystals  formed  by 
vacuum  distillation.  Both  kinds  of  powder  gave  the  same  results. 

The  powder  was  mounted  in  a  2-mm.  tube  of  thin  glass,  and  exposed  under  exactly 
the  same  conditions  as  silicon  and  aluminium.  Fig.  10  shows  a  photograph  obtained 
from  a  6-hour  exposure  at  32,000  volts  and  9  milliamperes,  and  Table  VIII  gives  the 
numerical  data. 

The  "theoretical  spacings"  in  Table  VIII  are  those  of  a  hexagonal  lattice  composed 
of  two  sets  of  triangular  prisms,  each  of  side  3.22  A.  and  axial  ratio  1.624,  with  construc- 
tion points  000  and  ^,  %,  ^  respectively.  This  is  the  lattice  whose  spacings  are  given 
in  column  5  of  Table  IV,  under  "Close-Packed  Lattice."  It  is  slightly  distorted,  how- 
ever, from  true  hexagonal  close  packing,  which  requires  an  axial  ratio  of  1.633.  This 
variation  from  theoretical  close  packing  is  to  be  attributed  to  a  slight  asymmetry  in  the 
structure  of  the  magnesium  atoms. 

The  agreement  between  calculated  and  experimental  spacings  is  satisfactory,  except 
that  several  lines  which  were  to  be  expected  do  not  show  in  the  photograph.  In  partic- 
ular, the  reflection  from  the  basal  plane,  0001,  is  absent  in  all  the  photographs. 

It  seemed  desirable,  therefore,  to  supplement  the  evidence  furnished  by  the  powder 
photographs  by  photographs  of  single  crystals,  mounted  with  definite  orientations. 
Several  such  photographs  were  taken,  the  measurements  of  three  of  which  are  given  in 
Table  IX.  The  crystals  were  formed  by  vacuum  distillation,  and  were  about  2mm.  in 
diameter.  The  first  was  mounted  with  its  basal  plane  (0001)  parallel  to  the  rays,  and 
rotated  slowly  about  an  axis  normal  to  1210,  for  about  30  deg.  on  each  side  of  the 
center.1 

The  second  crystal  was  mounted  so  as  to  rotate  about  the  same  axis  as  the  first,  but 
with  lOlO  parallel  to  the  rays  at  the  start.  The  third  was  mounted  with  1120  parallel 

TABLE  IX 


CRYSTAL    1 


CRYSTAL   2 


CRYSTAL   3 


Position 
of  Line 

Spacing 
of  Plane 

Indices  of  Plane 

Position 
of  Line 

Spacing 
of  Plane 

Indices  of  P.ane  ;  ™$£ 

Spacing 
of  Plane 

Indices  of  Plane 

3.10 

2.59 

0001 

2.90 

2.75 

1010 

5.02 

1.60 

1120 

2.90 

2.75 

1010 

3.10 

2.59 

0001 

5.50 

1.48 

1021 

3.30 

2.44 

1011 

3.30 

2.44 

1011 

6.0 

1.36 

1122 

4.20 

1.90 

1012 

6.05 

1.34 

2021 

5.50 

1.48 

1013 

8.92 

0.92 

1010(3) 

6.27 

1.30 

0001(2) 

5.9 

1.38 

1010(2) 

1  The  reflection  from  1010  should  not  have  appeared  on  this  plate.  It  was  very  faint,  but  clearly  visible  on  both  sides, 
and  was  probably  due  to  a  twin  upon  1011,  a  small  portion  of  which  was  included  in  cutting  the  crystal  from  the  mass  of 
other  crystals  upon  which  it  grew.  A  second  specimen  mounted  and  photographed  in  the  same  manner  did  not  show  this 
line.  Similar  twinning  must  account  for  the  0001  reflection  shown  by  crystal  2,  and  1013  "by  crystal  3. 


198 


to  the  rays,  and  rotated  about  an  axis  normal  to  1010.  The  patterns  obtained  in  these 
photographs  differed  from  those  of  the  single  iron  crystals  in  containing  reflections  from 
many  more  planes,  corresponding  to  the  greater  complexity  of  the  hexagonal  system. 
Molybdenum  rays  were  used,  filtered  through  .037  cm.  of  zircon,  so  that  the  spectrum 
consisted  of  a  single  line. 

The  lines  reflected  in  the  horizontal  plane,  which  appeared  on  the  three  photo- 
graphs, are  given  in  Table  IX.  The  0001  reflection  was  very  strong  on  the  first  photo- 
graph, and  on  three  additional  photographs  which  were  taken  to  make  certain  its 
identity.  Its  absence  in  all  the  powder  photographs  must  be  due,  therefore,  to  the 
much  greater  relative  intensity  of  the  reflections  from  other  forms,  containing  many 
more  planes.  These  forms  reflect  not  only  the  lines  but  the  unabsorbed  part  of  the  gen- 
eral spectrum,  causing  a  fog  over  the  plate  that  obscures  weak  lines. 

The  lines  tabulated  in  Table  IX,  and  all  the  others  which  appeared  on  these  photo- 
graphs, are  the  ones  which  should  appear,  with  the  exceptions  mentioned  in  the  above 
note. 

The  evidence  seems  sufficient  that  the  assumed  structure  is  correct,  viz.,  that  the 
atoms  of  magnesium  are  arranged  on  two  interpenetrating  lattices  of  triangular  prisms, 
each  of  side  3.22  A.  and  height  5.23  A.,  with  one  atom  at  each  corner,  the  atoms  of  one 
set  being  in  the  center  of  the  prisms  of  the  other. 

SODIUM 

The  first  photographs  of  sodium  were  of  rods,  about  1  mm.  in  diameter  and  1  cm. 
long,  cut  from  an  old  sample  that  had  been  in  the  laboratory  several  years.  These  rods 
were  placed  in  sealed  glass  tubes,  and  exposed  to  molybdenum  rays.  They  gave  intense 
reflections,  of  a  pattern  which  indicated  that  the  lump  from  which  the  samples  were 
cut  was  a  single  large  crystal. 

Several  unsuccessful  attempts  were  then  made  to  obtain  finely  divided  crystals  of 
sodium.  Distillation  in  vacuum  into  the  thin-walled  tube  which  was  to  be  photo- 
graphed was  found  impossible.  Several  different  glasses  and  pure  silica  tubing  were 
tried.  The  sodium  always  ate  through  the  tube  wall  before  it  could  be  coaxed  into  the 
narrow  tube.  Melting  the  distilled  sodium  so  that  it  flowed  into  the  tube  resulted  in  an 
amorphous  condition,  which  gave  no  lines  at  all.  It  is  probable  that  distillation,  had  it 
succeeded,  would  have  given  the  same  result,  for  potassium  distilled  in  this  way  was 
found  to  be  completely  amorphous.  Shaking  in  hot  xylol  gave  a  beautiful  collection  of 
tiny  spheres,  but  these  too  were  amorphous,  and  annealing  for  16  hours  at  90  deg.  C. 
failed  to  produce  any  appreciable  crystallization.  Crystallization  from  ammonia 
solution  gave  a  black  mass,  from  which  it  was  difficult  to  separate  the  pure  sodium. 
Fairly  good  photographs  were  obtained  with  fine  shreds,  scraped  from  the  lump,  with  a 
knife,  under  dry  zylol,  and  packed  in  a  small  glass  tube. 

A  satisfactory  sample  was  finally  prepared  by  squirting  the  cold  metal  through  a 
0.01  cm.  die,  and  packing  the  fine  thread,  with  random  folding,  into  a  1  mm.  glass  tube, 
which  was  immediately  sealed.  The  sample  from  which  this  was  taken  was  about  two 
months  old,  and  was  apparently  only  slightly  crystallized,  so  that  only  a  few  lines  were 
visible,  on  the  dense  continuous  background  due  to  the  amorphous  part.  Two  photo- 
graphs, taken  under  the  same  conditions  as  the  preceding,  with  exposures  of  4  and  14 
hours  respectively  at  30  kv.  27  milliamperes,  gave  identical  lines,  which  are  tabulated 
in  Table  X. 

199 


TABLE  X 

Sodium 

Intensity  of 

Distance  of 
Line  from 

Angular 
Deviation  of 

SPACING  OF  PLANES  IN 

Line 

Center 

Line  29 

ANGSTROMS                                                                        Number  of 

Theoretical 

J.llU.i^C3    UL     1    Ul  111 

X^UUlJCIclUIlg 

Planes 

Estimated 

Cm.                     Degrees 

Experimental  !       (Centered 

Cube) 

1.00 

2.66 

13.38 

3.05 

3.04 

110 

6 

.10 

3.78 

19.06 

2.15 

2.15 

100 

3 

.40 

4.66 

23.36 

1.76 

1.76 

211 

12 

.10 

5.38 

27.04 

1.52 

1.52                    110(2) 

6 

.08 

6.03               30.26 

1.36                 1.36                   310 

12 

.02 

6.57               33.40 

1.24                  1.24                    111 

4 

.05 

7.18              36.0 

1.15                 1.15                   321 

24 

These  seven  lines,  which  are  the  only  ones  that  appeared  on  any  of  the  sodium  photo- 
graphs, agree  perfectly  with  the  theoretical  spacings  of  a  centered  cubic  lattice,  of  side 
4.30  A.,  and  cannot  be  made  to  fit  any  other  simple  type  of  lattice. 
The  number  of  atoms  per  elementary  cube  is 


n  = 


0.970  X^ 


=  2.03, 


22.8X1.663 
which  is  as  close  to  the  required  number,  two,  as  the  data  would  warrant. 

The  evidence  is  sufficient,  I  think,  in  spite  of  the  limited  number  of  lines,  to  show 
that  the  atoms  of  sodium,  when  in  its  'crystalline  form,  are  arranged  on  a  lattice  whose 
unit  of  structure  is  a  centered  cube,  of  side  4.30  A.,  with  one  atom  at  each  corner  and  one 
in  the  center  of  the  cube.  The  tendency  to  form  this  regular  arrangement  is,  however, 
very  slight,  corresponding  to  a  small  difference  between  the  potential  energies  of  the 
crystalline  and  amorphous  states.  This  fact  is  important  for  the  determination  of  the 
structure  of  the  sodium  atom. 

The  structures  of  the  elements  thus  far  described  have  been  determined  with  con- 
siderable certainty.  The  three  following  have  been  only  partially  determined,  but  are 
included  as  examples  of  the  possibilities,  as  well  as  the  difficulties,  of  the  analysis. 

LITHIUM 

The  structure  of  lithium  is  of  special  interest  because,  on  account  of  the  small  num- 
ber of  electrons  associated  with  each  atom,  it  may  be  expected  to  yield  valuable  in- 
formation regarding  the  arrangement  of  these  electrons  around  the  nucleus.  The 
analysis  is  difficult,  however,  on  account  of  the  complete  lack  of  crystallographic  data, 
the  slowness  of  crystallization,  and  the  difficulty  of  obtaining  pure  metal. 

It  was  first  attempted  to  distill  lithium  in  vacuum,  for  the  double  purpose  of  purifi- 
cation and  of  obtaining  small  crystals.  Various  methods  of  heating  the  metal  were 
tried,  such  as  a  tungsten  spiral  with  the  lithium  ribbon  lying  in  its  axis,  a  molybdenum 
cup  heated  externally  by  electron  bombardment,  etc.,  but  without  success.  The  metal 
reacts  violently  with  glass  and  silica  at  temperatures  far  below  those  at  which  its  vapor 
pressure  is  appreciable. 

Two  samples  were  used.  The  first  was  prepared  by  electrolysis  of  pure  lithium 
chloride,  in  a  graphite  crucible,  and  probably  contains  little  impurity  except  carbon.  A 
small  lump  was  rolled,  between  steel  surfaces,  into  a  cylinder  2  mm.  in  diameter,  and 
sealed  in  a  glass  tube.  It  was  exposed,  in  the  same  manner  as  the  previous  crystals,  for 
7  hours  to  molybdenum  rays  at  40  kv.  and  6  milliamperes,  and  gave  the  lines  tabulated 
in  Table  XL 


200 


TABLE  XI 

Lithium 


Intensity  of 
Line 

Estimated 

Distance  of 
Line  from 

Angle  of 
Reflection 

SPANING  OF  PLANES  IN 
ANGSTROMS 

Indices  of  Form 

Xumber  of 
Cooperating 
Planes 

Cm 

Degrees 

Experimental     ^^ 

3.50 

100 

3 

.70 

3.20 

16.50                 2.50 

2.48 

110 

6 

1.00 

3.96 

20.44 

2.02 

2.02 

111 

4 

.05 

4.61 

23.4                   1.75 

1.75 

100(2) 

3 

1.56 

210 

12 

.40 

5.62 

29.0                   1.43 

1.43 

211 

12 

.02 

6.5 

33.6                   1.24 

1.24 

110(2) 

6 

.60 

6.95 

35.9                   1.17 

1.17 

/221 
\  100(3) 

15 

1.10 

310                             12 

1.05 

311                             12 

.10 

8.07 

41.6                   1.01 

1.01 

111(2)                         4 

.97 

320 

12 

.87 

100(4) 

3 

OC 

/410 

.85 

\  322 

24 

.05 

9.99 

51.6                     .83 

.83 

/411 
\  110(3)                         18 

.80 

331                             12 

.20 

10.82               56.0 

.77                   .78 

210(2)                        12 

The  spacings  calculated  are  those  of  a  simple  cubic  lattice,  of  side  3.50  A.,  and  the 
density  of  lithium  requires  that  2  atoms  be  associated  with  each  point  of  the  lattice, 
viz.: 


M     6.89X1.663 

A  centered  cubic  lattice  in  which  half  of  the  atoms,  those  belonging  to  one  of  the  two 
component  simple  cubic  lattices,  are  oriented  oppositely  to  the  other  half,  could  prob- 
ably be  made  to  fit  the  observations  by  assuming  a  suitable  arrangement  of  the  elec- 
trons in  the  atoms.  It  is  more  probable,  however,  in  view  of  the  next  photograph,  that 
the  strong  lines  at  3.96  cm.  and  6.95  cm.  are  due  either  to  an  impurity  or  to  the  ad- 
mixture of  a  second  form  of  lithium.  All  the  other  lines  in  Table  XI  are  consistent  with 
a  centered  cubic  lattice,  of  side  3.50  A.,  with  one  atom  of  lithium  at  each  cube  corner 
and  one  in  the  center  of  each  cube. 

The  second  sample  was  taken  from  a  very  old  stock  of  supposedly  very  pure 
lithium,  origin  not  known.  A  fine  thread  was  squirted  through  a  die  and  packed  into  a 
glass  tube,  in  the  same  manner  as  sodium.  A  five-hour  exposure  to  molybdenum  rays, 
at  30  kv.  27  milliamperes,  gave  only  3  lines,  viz.,  a  strong  line  at  3.22  cm.,  and  two 
weaker  lines  at  4.60  cm.,  and  5.46  cm.  These  are  exactly  the  positions  of  the  first  three 
lines  of  the  centered  cubic  lattice  described  above,  and  it  is  especially  noteworthy  that 
the  line  at  4.60  is  relatively  much  stronger  than  on  the  preceding  photograph,  and  the 
strong  lines  at  3.96  cm.  and  6.95  cm.  are  entirely  lacking.  One  is  tempted  to  consider  this 
last  photograph  as  that  of  pure  lithium,  since  its  interpretation  is  simpler  than  the  pre- 
ceding, and  it  gives  to  lithium  the  same  structure  as  sodium.  The  number  of  lines  is  too 
small,  however,  to  justify  this  conclusion,  and  further  experiments  with  purer  metal  are 
needed. 

NICKEL 

Specially  purified  nickel  wire  was  melted  in  vacuum  and  cast  in  a  lump.  Filings 
from  this  lump  were  placed  in  a  small  cell  2.5  mm.  thick,  and  exposed  4  hours  to  tung- 


201 


sten  rays,  produced  at  110,000  volts,  1  milliampere,  filtered  through  0.015  cm.  of  tan- 
talum. The  photographic  plate  was  placed  -15.7  cm.  from  the  crystal,  at  right  angles  to 
the  beam  of  X-rays.  The  lines  obtained  are  tabulated  in  Table  XII. 


TABLE  XII 

Nickel 


Distance  of 
Intensity  of                      Line  from 

Lme                               Centre 

Angular 
Deviation  of 
Line 

SPACING  OF  PLANES  IN 
ANGSTROMS 

Indices  of 
Form 

Number  of 
Cooperating 
Planes 

Estimated                             Cm. 

Degrees 

Experimental 

Theoretical 
(Centered 
Cube) 

Very  strong  1.70 

6.17 
8.75 
10.70 
12.25 
13.60 

16.28 

1.95 
1.38 
1.13 
.98 
.89 

.74 

1.95 
1.38 
1.13 
.98 
.87 
.79 
.74 

110 
100 
211 
110(2) 
310 
111 
321 

6 
3 
12 
6 

12 
4 
24 

Faint  2.42 

Strong  2.97 

Medium  3.42 

Faint  3.80 

Strong   .-  .  .           4.58 

The  spacings  agree  perfectly  with  those  of  a  centered  cubic  lattice,  of  side  2.76  A. 
with  one  atom  of  nickel  at  each  cube  corner  and  one  in  the  center  of  the  cube.  Taking 
the  density  of  pure  nickel  as  9.00,  which  is  probably  too  low,  the  number  of  atoms  as- 
sociated with  each  elementary  cube  is 

9.00X2.76* 


_ 


M     58.2X1.663 
which  is  as  close  to  the  required  value,  2,  as  the  data  will  warrant. 

Three  other  photographs,  one  of  a  thick  electrolytic  deposit  on  very  thin  nickel  foil, 
the  other  two  of  a  2  mm.  nickel  rod  of  unknown  origin,  gave  quite  different  lines.  The 
electrolytic  deposit  was  exposed  but  a  short  time,  and  gave  4  lines,  at  1.64,  1.89,  2.70 
and  3.14  cm.  corresponding  to  spacings  of  2.10,  1.76,  1.25,  1.07  respectively,  which  are 
exactly  the  spacings  of  the  first  four  lines  of  a  face-centered  cube,  of  side  4-52  A.  The 
number  of  atoms  associated  with  the  elementary  cube  is 


9X3. 523 


=  4.02 


58.2X1.663 

which  is  correct  for  a  face-centered  cubic  lattice  containing  one  atom  of  nickel  at  each 
corner  and  one  in  the  center  of  each  face. 

The  other  two  photographs,  of  the  nickel  rod  of  unknown  origin,  contained  the  lines 
of  both  the  preceding  ones,  but  only  these  lines.  It  was  presumably  a  mixture  of  the 
two  crystalline  forms  of  nickel,  represented  by  the  two  preceding  specimens  respectively. 

The  evidence  is  very  strong,  therefore,  that  nickel  crystallizes  in  two  different  forms, 
one  a  centered  cubic  lattice,  like  iron,  and  the  other  a  face-centered  cubic  lattice,  like 
copper.  The  relation  of  the  magnetic  and  mechanical  properties  to  these  crystalline 
changes  has  not  been  studied,  and  the  above  analysis  is  to  be  regarded  as  only  pre- 
liminary. 

GRAPHITE 

Several  photographs  of  both  natural  and  artificial  graphite  have  been  taken.  The 
natural  graphite  was  in  large  flakes,  obtained  from  the  Dixon  Crucible  Company.  The 
artificial  graphite  was  a  very  fine  powder,  furnished  by  the  Acheson  Company.  Both 
had  been  heated  to  3500  deg.  C .  in  a  special  graphite  furnace  to  remove  impurities  and  ash . 

The  natural  graphite,  either  in  large  flakes  or  where  pressed  into  a  glass  tube,  gave 
very  unsymmetrical  photographs,  showing  the  predominance  of  certain  orientations  of 
the  crystals.  By  forcing  it  through  a  copper  gauze  of  100  meshes  to  the  inch,  a  power 
was  obtained  which,  when  packed  in  a  glass  tube  and  kept  in  rotation,  gave  very  regular 

202 


TABLE  XIII 
Graphite 


Intensity 
of  Line 

Distance  of 
Center 

Angular 
Deviation  of 
Line 

SPACING  OF  PLANES  IN 
ANGSTROMS 

Indices  of  Form 

Number  of 
Cooperating 
Planes 

Estimated 

Cm. 

Degrees 

Experimental 

Theoretical 

100 

2.40 

12.16 

3.37 

3.37 

0001 

1 

30 

3.84 

19.46 

2.11 

2.12 

loio 

3 

60 

3.99 

20.20 

2.03 

2.02 

1011 

3 

1 

4.47 

22.60 

1.81 

1.80 

1012 

6 

3 

4.81 

24.34 

1.690 

1.685 

0001(2) 

1 

2 

5.21 

26.38 

1.560 

1.544 

1013 

6 

1.318 

1014 

6 

35 

6.65 

33.70 

1.227 

1.227 

1120 

3 

50 

7.09 

35.90 

1.155 

1.152 

1122 

6 

1.138 

1015 

6 

1.124 

0001(3) 

1 

1.062 

1010(2) 

3 

3 

7.82 

39.60 

1.050 

1.048 

2021 

6 

1.008 

1011(2) 

6 

15 

8.31 

42.10 

.990 

J.994 
1.990 

1016 
1124 

6 
6 

.960 

2023 

6 

.897 

1012(2) 

6 

.877 

1017 

6 

.842 

0001(4) 

1 

.833 

2025 

6 

2 

10.06 

51.0 

.827 

.829 

1126 

6 

5 

10.41 

52.8 

.800 

/  .802 
1.797 

2130 
2131 

6 
12 

.783 

1018 

6 

.780 

2132 

12 

.773 

1013(2) 

6 

.756 

2133 

12 

.725 

2134 

12 

.715 

2027 

6 

7 

11.85 

60.0 

.712 

/.708 
1.708 

1010(3) 
1019 

3 
6 

15 

12.11 

31.4 

.697 

f  .696 

\.693 

1128 
3032 

6 
6 

.690 

2135 

12 

1 

12.61 

64.0 

.672 

f  .674 
1.674 

1011(3) 
0001(5) 

6 

1 

.660 

1014(2) 

6 

9 

12.94 

65.6 

.656 

/.654 
1.654 

3034 
2136 

6 
12 

.644 

10110 

6 

1 

13.79 

69.8 

.621 

/.616 
1.616 

2137 
1120(2) 

12 
3 

3 

14.11 

71.50 

.609 

/.612 
1.612 

2029 

2241 

6 
6 

.603 

1121(2) 

6 

.598 

1012(3) 

6 

2 

14.57 

73.8 

.592 

/  .592 
1.592 

2243 
11210 

6 
6 

and  symmetrical  photographs.  The  lines  in  these  photographs  were  identical  with  those 
in  the  photographs  of  artificial  graphite,  showing  that  the  two  are  identical  in  crystalline 
structure.  One  of  these  photographs,  obtained  from  a  16-hour  exposure  to  Mo  rays  at 


203 


34,000  volts  and  16  milliamperes,  is  reproduced  in  Fig.  11,  and  the  lines,  together  with 
the  calculated  spacings,  are  tabulated  in  Table  XIII. 

The  crystallographic  data  regarding  graphite  is  very  meager  and  uncertain,  and  in 
attempting  to  guess  its  crystalline  structure  one  has  an  embarrassing  freedom  of  choice, 
both  of  crystal  systems  and  of  axial  ratios  and  angles.  The  only  guiding  principles, 
apart  from  the  lines  in  the  photograph  are,  first,  that  the  true  structure  is  probably  very 
simple  and  symmetrical,  since  all  its  atoms  are  alike,  and  second,  that  the  nearest  ap- 
proach of  adjacent  atoms  cannot  be  very  different  from  that  in  diamond. 

The  structure  whose  spacings  are  tabulated  in  Table  XIII,  fits  the  experimental 
data  best  of  all  that  have  been  tried,  and  seems  capable,  when  account  is  taken  of  the 
internal  structure  of  the  atoms,  of  explaining  all  the  observed  intensities  of  the  lines. 
It  is  a  hexagonal  structure,  composed  of  four  simple  lattices  of  triangular  prisms,  each  of 
side  2.47  A.  and  height  6.80  A.,  the  atoms  of  the  third  lattice  being  directly  above  those 
of  the  first  at  a  distance  of  one  half  the  height  of  the  prism,  those  of  the  2d  and  4th 
lattices  being  above  the  centers  of  alternate  triangles  of  the  first,  at  distances  1/14  and 
8/14  respectively  of  the  height  of  the  prism.  The  co-ordinates  of  the  atoms  are: 

m,  n,  pc, 


m,  n, 


where  m,  n,  and  p  have  all  possible  values  and  c,  the  axial  ratio,  is  2.75.  The  0001 
planes  are  thus  arranged  in  pairs,  similar  to  the  111  planes  in  diamond.  The  distance 
between  nearest  consecutive  planes,  and  between  atoms  in  each  plane,  .48  A.  and  2.47 
A.  respectively,  are  slightly  less  than  their  values  .51  and  2.52  for  diamond,  and  the 
nearest  approch  of  atoms  is  1.50  A.  as  compared  to  1.54  for  diamond.  This  closer  ap- 
proach of  the  atoms  in  graphite  would  indicate  chemical  stabilty.  The  distance  be- 
tween consecutive  pairs  of  planes,  however,  is  much  greater,  viz.,  3.40  A.  in  graphite, 
than  its  value  2.06  A.  in  diamond,  which  accounts  for  the  extreme  ease  of  basal  cleavage 
and  gliding  in  graphite. 

The  agreement  between  experimental  and  calculated  spacings  in  Table  XIII  is  well 
within  the  limit  of  the  experimental  error,  which  is  about  1  per  cent.  Every  experi- 
mental spacing  is  accounted  for,  the  first  12  with  certainty,  the  last  7  with  some  am- 
biguity on  account  of  the  large  number  of  theoretical  spacings.  The  absence  of  re- 
flection from  planes  such  as  1014,  1015,  the  second  orders  of  1010  and  1011,  and  the 
third  and  fourth  orders  of  0001,  is  dependent  not  only  on  the  positions,  but  on  the 
internal  structure  of  the  atoms,  and  cannot  be  interpreted  except  in  conjunction  with 
a  study  of  this  internal  structure,  which  will  be  undertaken  as  soon  as  accurate 
photographic  measurements  can  be  obtained. 

The  structure  given  above  has  the  lowest  symmetry  of  any  elementary  substance 
yet  studied.  It  may  be  that  the  essential  elements  of  the  hexagonal  lattice  can  be  more 
simply  represented  by  a  monoclinic  or  triclinic,  or  possibly  an  orthorhombic  lattice, 
though  efforts  in  this  direction  have  so  far  been  unsuccessful. 

DIAMOND 

The  crystal  structure  of  diamond  has  been  completely  determined  by  the  Braggs,1 
and  confirmed  by  numerous  observers.  Comparison  of  the  results  of  these  investiga- 
tors with  those  obtained  from  a  powder  photograph  will  therefore  serve  as  an  excellent 
check  upon  the  latter.  In  addition,  the  powder  photograph  of  diamond  has  a  merit  of 
its  own,  for  it  furnishes  evidence  not  hitherto  available  regarding  the  internal  structure 

1  X-Rays  and  Crystal  Structure,  p.  102  ff.,  Proc.  Roy.  Soc.  A.,  89,  277. 

204 


of  the  most  interesting  of  all  atoms.  The  photographs  taken  thus  far  are  not  suitable 
for  photometering,  but  arrangements  are  complete  for  taking  such  photographs,  and  for 
measuring  the  intensity  of  the  lines. 

Several  photographs  of  diamond  have  been  taken,  under  varying  conditions,  with 
identical  results,  as  regards  position  and  relative  intensity  of  lines.  Fig.  12  shows  the 
result  of  a  fifteen-hour  exposure  to  Mo  rays  at  30,000  volts,  35  milliamperes,  with 
zircon  filter  of  0.37  mm.  A  very  thin  wall  glass  tube  of  special  lithium  boro-silicate 
glass,  2  mm.  in  diameter,  was  filled  with  diamond  powder,  obtained  by  crushing  some 
old  dies  in  a  steel  mortar.  This  powder  was  mounted  on  the  spectrometer  table,  con- 
centric with  a  wooden  disc  10.27  cm.  in  diameter,  upon  which  Eastmen  X-ray  film  was 
fastened  in  a  complete  circle,  except  for  a  5  mm.  hole  where  the  rays  entered.  The 
collimator  slits  were  about  1.5  mm.  wide,  and  the  distance  from  X-ray  target  to  powder 
was  approximately  35  cm.  Only  one  half  of  this  film,  corresponding  to  angles  of 
diffraction  from  0  deg.  to  180  deg.,  is  shown  in  Fig.  12.  Twenty-five  of  the  possible  27 
lines  are  visible  in  the  photograph,  the  last  two  being  obscured  by  the  dense  fog. 

TABLE  XIV 


Intensity  of 
Line 

Distance  of 
Line  from 
Center  x 

Angular 
Deviation  of 
Line  20 

Degrees 

SPACING  OF  PLANES  IN  ANGSTROMS 

Indices  of  Form 

Number  of 
Cooperating 
Planes 

Estimated 

Cm. 

Experimental 

Theoretical 

1.00 

1.80 

20.06 

2.05 

2.06 

111 

4 

.50 

2.96 

33.0 

1.26 

1.26 

110 

6 

.40 

3.49 

39.92 

1.072 

1.075 

311 

12 

.10 

4.26 

47.4 

.885 

.890 

100 

3 

.25 

4.66 

52.0 

.813 

.817 

331 

12 

.40 

5.31 

59.2 

.721 

.728 

211 

12 

.20 

5.66 

63.0 

.680 

.683 

[111(3) 
1  511 

16 

.10 

6.22 

69.4 

.625 

.630 

110(2) 

6 

.20 

6.54 

73.0 

.597 

.602 

531 

24 

.15 

7.10 

79.2 

.558 

.563 

310 

12 

.06 

7.43 

82.8 

.538 

.543 

533 

12 

.03 

7.98 

89.0 

.507 

.513 

111(4) 

4 

.08 

8.24 

91.8 

.496 

.498 

/711 

\551 

24 

.20 

8.76 

97.6 

.473 

.476 

321 

24 

.15 

9.06 

101.0 

.462 

.463 

[  731 
\  553 

36 

.005 

9.70 

107.6 

.442 

.445 

100(4) 

3 

.003 

10.00 

113.2 

.432 

.435 

733 

12 

.12 

10.52 

116.8 

.417 

.420 

/411 
1  110(3) 

18 

.08 

10.84 

120.8 

.409 

.411 

J  751 
\  111(5) 

28 

.05 

11.50 

127.6 

.397 

.397 

210 

12 

/.08 
1  .02 

11.84 
11.93 

132.0  1 
132.8  J 

.389 

.391 

/  753 
\911 

36 

J  .05 
1.01 

12.54 
12.70 

139.8  1 
141.4  J 

.378 

.379 

332 

12 

J  .05 
1.01 

13.00 
13.23 

145.0  1 
147.2  J 

.372 

.373 

931 

24 

).07 
\.02 

14.00 
14.27 

156.0  1 
159.0  j 

.363 

.363 

211(2) 

12 

[933 

/.20 
\.06 

14.83 
15.35 

165.4  \ 
171.2  / 

.358 

.358 

J  755 

1  771 

48 

1311(3) 

The  lines  and  the  corresponding  spacings  are  tabulated  in  Table  XIV,  and  com- 
pared with  those  required  for  the  lattice  which  has  been  assigned  to  diamond  by  the 
Braggs.  The  agreement  is  absolute.  It  will  be  noted  that  for  the  larger  deviations  the 
doublet  of  the  molybdenum  radiation  is  clearly  resolved. 

205 


[CONTRIBUTION  FROM  THE  RESEARCH  LABORATORY  OF  THE  GENERAL  ELECTRIC  Co.] 
A  NEW  METHOD  OF  CHEMICAL  ANALYSIS* 

BY  A.  W.  HULL 

Received  April  16,  1919 

Two  methods  of  X-ray  chemical  analysis  are  already  fairly  well  known. 

The  first,  which  may  be  called  X-ray  spectrum  analysis,  is  the  result  of  the  classical 
experiments  of  Moseley,1  and  consists  in  attaching  the  substance" to  be  investigated  to 
the  target  of  an  X-ray  tube  and  photographing  its  X-ray  line  spectrum.  The  X  lines 
belonging  to  each  element  are  very  few  in  number  as  compared  with  the  very  large  num- 
ber in  the  visible  spectrum,  and  they  bear  a  simple  relation  to  the  atomic  numbers  of 
the  elements,  so  that  they  can  be  identified  quickly  and  absolutely  by  comparison  with 
standard  tables.  Moseley's  measurements  have  been  extended  by  Siegbahn2  and  his 
collaborators  to  practically  all  known  elements  of  atomic  weight  greater  than  sodium, 
and  the  lines  have  been  collected  in  a  table  for  convenient  reference.2  This  method  is 
applicable  to  all  substances  which  can  be  attached  to  the  target  of  an  X-ray  tube,  ex- 
cept those  in  the  first  row  of  the  periodic  table.3  It  thus  supplements  visible  spectrum 
analysis  in  being  most  easily  applicable  where  the  latter  is  least  so,  viz.,  to  substances 
not  easily  volatilized. 

The  second  method,  which  may  be  called  the  X-ray  absorption  band  method,  is  due 
to  the  discovery  of  Barkla4  of  the  X-ray  absorption  bands  of  the  chemical  elements. 
The  position  of  the  edges  of  these  bands  have  now  been  measured  accurately  and 
tabulated  for  practically  all  the  elements  above  zinc5  by  Duane  and  Blake.  The 
method  of  analysis  consists  in  placing  the  substance  to  be  examined,  in  the  form  of 
an  absorbing  layer,  either  liquid  or  solid,  in  front  of  an  X-ray  tube,  photographing  the 
spectrum,  and  comparing  the  absorption  bands  in  the  photograph  with  the  tabulated 
values.  It  is  applicable  to  all  chemical  elements  except  those  in  the  first  row  of  the 
periodic  table. 

Both  of  these  methods  give  evidence  only  of  the  chemical  elements  present,  and  not 
of  their  state  of  chemical  combination.  Both  are  capable  of  quantitative  as  well  as 
qualitative  application.  They  have  the  advantage  over  older  methods  that  their  re- 
sults are  absolutely  unambiguous,  since  they  depend  only  on  the  atomic  numbers  of  the 
elements  in  question,  and  not  upon  any  of  their  chemical  properties  or  states  of  com- 
bination. The  fact  that  these  methods  have  not  as  yet  come  into  common  use  is  due 
not  so  much  to  any  difficulty  in  their  application,  as  to  the  fact  that  they  are  new,  and 
that  no  problem  of  sufficient  importance  has  presented  itself  to  warrant  their  rapid 
development. 

*  Copyright,  1919,  American  Chemical  Society. 

1  Moseley,  Phil.  Mag.,  26.  1024  (1913);  £7,  703  (1914). 

2  Siegbahn,  Jahrb.  Radioact.  Electronik,  13,  336  (1916). 

3  The  limitation  is  due  to  the  fact  that  no  crystal  is  known  with  atoms  far  enough  apart  to  act  as  a  grating  for  the 
relatively  long  wave  lengths  characteristic  of  these  first  row  elements. 

«  Phil.  Mag.,  17,  739  (1909). 
s  Phys.  Rev.,  10,  697  (1917). 

207 


The  purpose  of  this  paper  is  to  describe  a  third  and  fundamentally  different  method 
of  X-ray  chemical  analysis.  It  is  simpler  than  the  other  two  in  that  it  does  not  require 
a  spectrometer,  and  it  supplements  them  in  that  it  gives  evidence  which  they  do  not 
supply,  namely,  the  state  of  chemical  combination  for  each  of  the  elements  present. 

The  method  consists  in  reducing  to  powder  form  the  substance  to  be  examined, 
placing  it  in  a  small  glass  tube,  sending  a  beam  of  monochromatic  X-rays  through  it, 
and  photogrophing  the  diffraction  pattern  produced.  The  only  apparatus  required  is  a 
source  of  voltage,  an  X-ray  tube,  and  a  photographic  plate  or  film.  The  amount  of 
material  necessary  for  a  determination  is  one  cubic  millimeter.  The  method  is  applica- 
ble to  all  chemical  elements  and  compounds  which  are  crystalline  in  structure.1 

The  arrangement  of  apparatus  is  shown  in  Fig.  1 .  T  is  a  transformer  furnished  with 
an  extra  coil  for  lighting  the  filament  of  the  X-ray  tube ;  X  a  Coolidge  X-ray  tube ;  F  a 
sheet  of  metal,  properly  chosen,2  serving  as  a  filter;  Si  and  S2  slits  in  thin  sheets  of  lead; 
T  a  thin-walled  tube,  about  one  mm.  in  diameter,  of  some  light  amorphous  material, 
such  as  glass,  celluloid,  or  collodion,  containing  the  powdered  substance  to  be  tested; 
and  F  a  narrow  strip  of  photographic  film  bent  over  a  semicircular  strip  of  brass  or 
wood,  concentric  with  T.3 


Fig.   1 


The  rays  from  the  X-ray  tube  pass  first  through  the  filter,  which  absorbs  all  but  a 
single  wave  length;  then  through  the  two  slits,  which  confine  them  to  a  narrow  beam 
(about  one  mm.  wide);  then  through  the  powdered  material,  which  scatters  or  "re- 
flects" a  very  small  fraction  of  them;  and  thence  to  the  center  of  the  photographic 
film.  An  exposure  of  one  hour  will  generally  give  all  the  information  desired. 

When  the  film  is  developed  it  shows,  in  addition  to  the  over-exposed  line  in  the  cen- 
ter where  the  direct  beam  strikes,  a  series  of  other  lines  on  each  side  of  the  center. 
These  lines  are  caused  by  the  "reflections"  of  the  X-rays  from  the  tiny  crystals  in  the 
powder.  Their  distance  from  the  center  of  the  film  depends  on  the  distance  between 


1  The  number  of  non-crystalline  solid  substances  is  probably  very  small.     All  the  solids  thus  far  examined,  including 
many  that  have  been  considered  amorphous,  have  been  found  crystalline  with  the  single  exception  of  glass. 

2  The  filter  is  chosen  of  such  material  that  it  specially  absorbs  all  wave  lengths  shorter  than  the  desired  one,  leaving 
practically  nothing  but  a  single  intense  line,  the  a  line  of  the  K  series  of  the  anode  material.    The  proper  material  for  the 
filter  depends  upon  the  material  of  the  X-ray  tube  anode.    For  a  molybdenum  X-ray  tube  the  proper  material  is  zirconium. 
For  details  see  Phys.  Rev.,  10.  665  (1917). 

3  For  rapid  work  it  is  desirable  to  use  Dupli-Tized  X-ray  film,  and  place  on  each  side  of  it  a  thin  strip  of  calcium 
tunestate  intensifying  screen. 


208 


Fig.  2 

the  planes  of  atoms  in  the  crystal,  and  there  is  one  line  for  every  important  set  of 
planes  in  the  crystal.  It  is  evident,  therefore,  that  substances  with  different  crystalline 
structure  will  give  entirely  different  patterns  of  lines  (compare,  for  example,  silicon, 
magnesium  carbonate,  lithium  fluoride,  Fig.  4).  Substances  of  similar  chemical  nature, 
on  the  other  hand,  will  in  general  have  similar  crystal  structure,  and  give  similar  patterns, 
so  that  is  it  often  possible  to  identify  a  photograph  at  a  glance  as  belonging  to  a  certain 
type  of  element  or  compound.  Thus,  lithium,  sodium  and  "potassium  fluorides,  sodium 
and  potassium  chlorides,  and  magnesium  oxide  (Fig.  3)  all  have  the  same  arrangement 
of  atoms  in  their  crystals,  and  all  give  precisely  similar  patterns  of  lines,  the  one  being 
simply  a  magnified  image  of  the  other.  The  magnification  or  spread  of  the  pattern  is 
different  for  each  one,  being  inversely  proportional  to  the  cube  root  of  the  molecular 
volume.  Si'nce  no  two  similar  substances  have  exactly  the  same  molecular  volume,  it  is 
easy  to  distinguish  them,  as  the  difference  is  cumulative  for  lines  far  from  the  center.1 


LLLJ 


Fig.  3 


As  an  example,  the  photographs  of  potassium  and  sodium  chlorides,  which  are  the  near- 
est together  of  any  of  the  patterns  thus  far  investigated,  have  been  placed  side  by  side 
in  Fig.  3  for  comparison.  A  further  distinguishing  mark  is  the  relative  intensity  of 
the  different  lines,  which  differs  greatly  even  in  the  most  closely  related  compounds, 
depending  on  the  relative  shapes  and  sizes  of  the  atoms  in  the  compound.  Thus 
lithium  fluoride,  magnesium  oxide,  sodium  fluoride  and  potassium  chloride  have  pre- 
cisely similar  patterns  (Fig.  3),  but  certain  lines,  as  the  first  and  fourth,  are  very  strong 
in  lithium  fluoride,  fairly  strong  in  magnesium  oxide,  barely  visible  in  sodium  fluoride, 
and  entirely  lacking  in  potassium  chloride. 


1  The  lines  farthest  from  the  center  diverge  even  more  than  the  difference  jn  molecular  volume,  since  the  cube  root 
of  molecular  volume  is  strictly  proportional,  inversely,  to  the  sine  of  the  angles  of  reflection,  whereas  the  distances  of  the 
lines  from  the  center  are  proportional  to  the  angles  themselves.  The  difference  is  negligible  for  lines  near  the  center 
(small  angles),  but  for  large  angles  the  dispersion  thus  produced  is  very  large,  so  that  two  exactly  similar  substances 
differing  in  molecu'ar  volume  by  less  than  1  per  cent  could  easily  be  distinguished. 

209 


Further  details  concerning  the  theory  of  the  production  of  these  lines,  and  their  re- 
lation to  the  crystalline  structure  of  the  substance,  will  be  found  in  the  Physical  Re- 
view.1 This  theory  will  not  be  reproduced  here,  as  is  it  not  essential  to  chemical 
analysis,  beyond  establishing  the  facts  that  every  crystalline  substance  gives  a  pattern; 
that  the  same  substance  always  gives  the  same  pattern ;  and  that  in  a  mixture  of  sub- 
stances each  produces  its  pattern  independently  of  the  others,  so  that  the  photograph 
obtained  with  a  mixture  is  the  superimposed  sum  of  the  photographs  that  would  be 
obtained  by  exposing  each  of  the  components  separately  for  the  same  length  of  time. 
This  law  applies  quantitatively  to  the  intensities  of  the  lines,  as  well  as  to  their  posi- 
tions, so  that  the  method  is  capable  of  development  as  a  quantitative  analysis. 

As  illustrations  of  the  general  type  of  photographs  obtained  with  simple  compounds 
and  elements,  Fig.  2  shows  two  typical  photographs,  of .  silicon  and  sodium  fluoride, 
respectively;  Fig.  3  a  series  of  isomorphous  alkali  halogens,  illustrating  their  similarity 
of  pattern  and  their  differences  in  spacing  and  intensity;  and  Fig.  4  gives  a  series  of 
dissimilar  substances,  illustrating  their  different  types  of  pattern. 

As  practical  examples,  two  actual  analyses  will  be  described.  They  are  only 
roughly  quantitative,  but  could  easily  be  refined  to  any  required  accuracy.  In  addi- 
tion, they  give  information  which  no  other  method  of  analysis  can  furnish. 


Fig.  4 

The  first  analysis  was  of  a  sample  of  sodium  fluoride,  taken  from  stock,  labelled 
"c.  P."  It  was  photographed  in  the  manner  described  and  gave  the  pattern  shown  in 
the  middle  section  of  Fig.  5.  A  sample  of  very  pure  sodium  fluoride  was  then  prepared 
and  photographed,  with  the  results  shown  in  the  lower  section  of  Fig.  5.  It  is  evident 
from  the  correspondence  of  the  lines  that  the  unknown  sample  was  sodium  fluoride, 
but  that  it  contained  a  large  amount  of  impurity,  which  one  would  estimate,  from  the 
relative  intensity  of  the  lines,  at  30  or  40  per  cent.2  In  order  to  determine  the  nature  of 
the  impurity,  a  series  of  photographs  was  taken  of  substances  which  were  considered 
the  most  probable  constituents,  such  as  sodium  carbonate,  sodium  chloride,  sodium 
hydrogen  fluoride,  etc.  The  pattern  of  sodium  hydrogen  fluoride  is  shown  in  the  upper 
section  of  Fig.  5.  It  is  evident  at  a  glance  that  it  corresponds  to  the  impurity  in  the 
the  test  sample  of  sodium  fluoride,  and  a  careful  examination  shows  that  all  the 
lines  not  common  to  the  two  lower  photographs  are  common  to  the  two  upper  ones. 
In  other  words,  sodium  hydrogen  fluoride  is  the  only  impurity  that  is  present  in  ap- 
preciable quantity.  The  amount  present  can  be  roughly  estimated  from  the  relative 
intensity  of  the  lines,  and  this  could  be  made  into  a  quantitative  method  by  preparing 
for  comparison  a  series  of  photographs  of  mixtures  of  know  composition. 

1  Phys.  Rev..  10,  661-696  (1917). 

2  This  sample  was  later  titrated  and  found  to  contain  19.2  per  cent  F,  which  corresponds  to  60  per  cent  NaHFj. 

210 


The  fact  to  be  emphasized,  however,  is  that  this  analysis  shows  that  the  sample  con- 
sisted of  a  simple  mixture  of  separate  crystals  of  sodium  fluoride  and  sodium  hydrogen 
fluoride,  and  not  a  mixture  of  these  with  a  hydrate,  or  some  more  complex  compound. 
Information  of  this  kind  might,  in  some  cases,  be  of  considerable  value,  and  it  can  al- 
ways be  obtained  by  this  method. 

The  second  example  is  the  analysis  of  two  samples  of  identical  chemical  content, 
viz.,  33.5  per  cent  potassium,  19.7  per  cent  sodium,  16.3  per  cent  fluorine,  and  30.5  per 
cent  chlorine.  The  photographs  given  by  these  two  samples  are  shown  together,  for 
comparison,  in  Fig.  6.  It  is  evident  that  the  two  samples  are  far  from  being  identical, 
in  fact,  that  they  contain  nothing  in  common. 


Fig.  5 


NaF-36% 
KCt  -6t% 
NaC  1-50.2'. 

K  F  -  nv 


Fig.  6 


The  first  of  these  photographs  is  shown  again  in  Fig.  7,  in  comparison  with  sodium 
fluoride  and  potassium  chloride,  and  is  seen  to  contain  all  the  lines  of  both  of  them,  and 
no  other  lines.  Hence  this  sample  consists  of  a  mixture  of  sodium  fluoride  and  potas- 
sium chloride  (36  per  cent  sodium  fluoride,  64  per  cent  potassium  chloride)  and  nothing. 
else.  To  show  how  conclusive  the  test  is,  this  same  sample  is  shown  again  in  Fig.  8  in 
comparison  with  sodium  chloride  and  potassium  fluoride.  It  is  evident  that  neither  of 
these  patterns  is  present  in  the  sample.  The  chance  correspondence  of  individual  lines 
has  no  meaning.  If  a  substance  is  present,  its  whole  pattern  must  be  present  in  the 
photograph,  and  the  relative  intensities  in  this  pattern  must  be  the  same  as[in  the  com- 
parison standard.  Thus  the  absence  in  the  test  photograph  of  a  single  intense  line  of 
comparison  substance  proves  that  none  of  this  substance  is  present,  and  that  any  other 
correspondences  between  the  two  are  mere  chance. 


Fig.  7 

211 


Fig.  8 


Fig.  9 


The  second  sample  is  shown  in  Fig!  9,  in  comparison  with  sodium  chloride  and 
potassium  fluoride,  and  it  is  evident  that  it  consists  of  a  mixture  of  these  two  salts  (50.2 
per  cent  sodium  chloride,  49.8  per  cent  potassium  fluoride)  and  nothing  else. 

These  examples  are  very  simple  ones.  It  is  possible  to  go  much  further.  By  nar- 
rowing the  slits  and  using  a  smaller  tube  of  test  material  very  sharp,  narrow  lines  can 
be  obtained,  and  a  mixture  of  several  substances  analyzed  without  ambiguity. 
Furthermore,  by  long  exposures,  so  as  to  greatly  overexpose  the  principal  components 
of  a  mixture,  substances  present  only  in  very  small  amounts  can  be  made  to  show. 


212 


THE  POSITIONS  OF  ATOMS  IN  METALS* 

BY  A.  W.  HULL 

The  determination  of  the  exact  positions  of  atoms  in  solid  bodies  is  the  next  to  last 
of  a  series  of  discoveries,  that  have  made  atoms  as  real  as  the  bricks  of  which  houses  are 
built. 

The  atom  of  20  years  ago  was  the  "hypothetical  smallest  sub-division  of  matter." 
The  atom  of  today  is  a  real  object  of  definite  shape  and  size.  We  know  what  it  is  made 
of.  We  know  its  weight  in  grams.  We  can  see  its  splash  when  it  impinges  on  a  plate 
of  fluorescent  material.  We  know  its  exact  speed  when  it  flies  about  as  gas.  And, 
lastly,  we  know  its  exact  position  when  it  forms  part  of  a  solid  body. 

A  brief  enumeration  of  these  discoveries  is  a  necessary  introduction  to  the  following 
discussion. 

First  came  the  discovery  of  dancing  molecules.  Heat  had  been  considered  a  sub- 
stance. The  "  Kinetic  Theory  of  Gases"  proved  that  it  is  a  condition,  viz.,  the  motion 
of  the  molecules,  which  fly  about  like  frenzied  bees,  bumping  against  each  other  and 
the  walls  of  their  enclosure.  Through  this  discovery,  all  the  store  of  facts  and  laws 
about  gases  can  be  correlated  by  the  single  picture  of  these  dancing  molecules.  We  be- 
lieve in  these  dancing  molecules  as  firmly  as  in  the  law  of  gravitation.  Whenever  we 
think  of  gas  we  see  dancing  molecules. 

The  next  discovery  was  J.  J.  Thomson's  streaming  electrons.  Our  text-books  taught, 
and  some  still  do,  that  electricity  is  not  a  fluid,  though  it  behaves  in  many  ways  like  one. 
Thomson  proved  that  electricity  is  a  fluid,  that  its  atoms  are  the  electrons  which  con- 
stitute the  atoms  of  matter,  and  that  it  flows  through  wires  just  as  water  flows  through 
pipes. 

Next  came  the  weighing  of  the  atom.  Faraday  showed  long  ago  how  to  determine 
the  weight  of  an  atom  in  terms  of  the  charge  it  carries  in  electrolysis.  There  remained, 
therefore,  only  the  measurement  of  this  "unit  charge,"  viz.,  the  charge  of  a  single 
electron,  by  Millikan,  to  give  the  exact  weight  in  grams  of  any  atom  that  can  be 
deposited  electrolytically.  As  soon  as  the  weight  of  any  one  atom  is  known,  the  weights 
of  all  the  others  can  at  once  be  calculated  from  the  known  relative  atomic  weights. 

Then  came  the  counting  of  individual  atoms.  This  began  with  Sir  William  Crookes' 
"spinthariscope,"  and  culminated  in  the  beautiful  experiments  of  Rutherford  and 
Geiger,  in  which  they  counted  one  by  one  the  helium  atoms  (the  so-called  "a  par- 
ticles") as  they  emerged  from  the  surface  of  disintegrating  radium;  and  then  allowed 
them  to  pass,  one  by  one,  into  a  thin-walled  glass  tube,  until  enough  had  accumulated 
to  form  a  gas  whose  pressure  could  be  measured  and  spectrum  analyzed. 

These  counting  experiments  led  directly  to  the  determination  of  the  composition  of 
the  atom.  J.  J.  Thomson  had  proved  that  every  atom  contains  electrons.  Rutherford 
proved  that  it  also  contains  a  positively  charged  kernel  or  nucleus,  very  small  compared 
to  the  whole  atom,  but  so  dense  that  it  contributes  nearly  the  whole  weight  of  the  atom. 
The  hypothetical  atom  thus  became  a  concrete  thing  that  can  be  visualized;  a  tiny 
(but  large  enough  to  be  studied)  solar  system,  with  nucleus  sun  and  elctron  planets. 
The  only  respect  in  which  one  kind  of  atom  differs  from  another  is  the  magnitude  of  the 


*  Copyright.  1919,  by  the  American  Institute  of  Electrical  Engineers. 

213 


positive  charge  of  the  nucleus,  which  determines  how  many  electrons  it  can  hold  in  its 
planetary  system,  and  hence  all  its  physical  and  chemical  properties. 

Finally  came  the  discovery,  by  the  Braggs,  of  the  method  of  determining  the  posi- 
tions of  the  atoms  in  solid  bodies.  The  beautiful  "point  lattices"  of  the  crystallo- 
graphers  were  hypothetical.  They  enumerated  possibilities,  but  could  not  point  out 
the  reality.  The  Bragg  measurements  of  atomic  distances  give  the  actual  arrange- 
ments. They  are  as  accurate  and  reliable  as  those  of  the  surveyor  or  astronomer. 
The  only  assumption  made  is  that  the  arrangement  of  atoms  is  a  regular  one  which 
repeats  itself,  and  this  assumption  can  be  checked  by  experiment.  The  method 
consists  simply  in  the  measurement,  by  means  of  a  special  "measuring  rod"  which 
will  be  described,  of  the  distance  between  atoms  in  three  or  more  different  directions. 
From  these  measurements  a  model  can  be  constructed,  which  can  then  be  checked 
by  further  measurements.  The  model  must  also  agree  with  known  physical 
properties  of  the  substance,  such  as  density,  atomic  weight,  and  crystal  habit. 
A  model  which  contains  but  one  kind  of  atoms  and  satisfies  all  these  tests  may  be 
regarded  as  very  reliable.  The  reliability  is  still  further  increased  by  the  fact  that 
all  the  models  investigated  thus  far  have  turned  out  to  be  very  simple.  In  cases 


Fig.  la.     Face  Centered  Cubic  Arrangement  (Cubic  Close  Packing) 

where  there  is  more  than  one  kind  of  atom,  i.e.,  compounds  or  alloys,  an  additional 
factor,  viz.,  the  size  and  shape  of  the  atoms,  must  be  taken  account  of.  There  is  one 
type  of  compound,  containing  only  two  kinds  of  atoms,  whose  structure  is  so  simple 
that  it  cannot  be  misunderstood.  Examples  of  this  type  will  be  included  in  the  follow- 
ing discussion.  Compounds  containing  more  than  two  kinds  of  atoms  have  not  yet 
been  sufficiently  studied  to  warrant  their  discussion,  but  there  is  every  reason  to  believe 
that  their  analysis  will  be  equally  simple  and  reliable. 

In  the  following  pages,  there  will  be  given,  first,  a  general  survey  of  the  results  ob- 
tained, then  a  brief  description  of  the  method  of  measurement,  and  lastly,  a  more  com- 
plete discussion  of  the  individual  models  and  some  of  their  properties. 

I  have  referred  to  the  location  of  atoms  as  next  to  last  in  the  series  of  atomic  dis- 
coveries. For  in  order  to  complete  the  picture,  one  more  discovery  is  necessary,  viz.,  the 
shape  and  size  of  the  atom.  An  excellent  beginning  in  this  direction  has  already  been 
made  by  Langmuir1  whose  theory  of  atomic  structure  predicts  the  shapes  and  relative 
sizes  of  all  the  atoms,  and  gives  strong  chemical  evidence  in  favor  of  these  predictions. 
The  author  hopes  soon  to  be  able  to  add  the  evidence  of  X-ray  measurements,  which 
will  determine  not  only  the  shape  but  the  exact  size  of  the  atoms,  that  is,  the  positions 
of  the  electrons  in  the  atoms. 


1  Langmuir,  J.  Amer.  Chem.  Soc.  41,  868,  June,  1919. 


214 


Summary  of  Results.  The  most  striking  result  of  these  investigations  is  the  ex- 
treme simplicity  of  arrangement  of  atoms  in  common  metals.  Among  the  metals  thus 
far  examined  only  three  types  of  atomic  arrangement  are  found,  and  these  are,  with  one 
exception,  the  three  simplest  geometrical  arrangements  known.  The  simplest  airange- 
ment  of  all  is  not  found  among  metals,  but  is  characteristic  of  salts,  which  are  composed 
of  equal  numbers  of  positive  and  negative  ions.  This  type  and  a  fifth  type,  also  very 
simple,  which  is  characteristic  of  non-metallic  elements,  will  be  included  in  the  dis- 
cussion for  the  purpose  of  comparison. 


Fig.  Ib.     Face  Centered  Cubic  Arrangement  (Cubic  Close  Packing) 

The  most  common  arrangement  in  metals  is  the  face  centered  cubic  arrangement, 
shown  in  Fig.  1 .  This  is  also  the  most  important  since  most  of  the  useful  metals, — e.  g., 
aluminium,  nickel,  cobalt,  copper,  silver,  platinum,  gold, — have  this  arrangement  of 
atoms.  Perhaps  it  would  be  better  to  say  that  those  substances  are  most  useful  as 
metals  which  have  this  arrangement,  since,  as  will  be  shown  later,  their  ductility  is  due 
largely  to  this  arrangement. 


Fig.  Ic.     Face  Centered  Cubic  Arrangement  (Cubic  Close  Packing) 

The  face-centered  cubic  arrangement  is  obtained  by  dividing  the  space  occupied  by 
a  single  crystal  or  "grain"  of  metal  up  into  a  system  of  equal,  closely  packed  cubes 
(Fig.  la)  and  placing  an  atom  at  each  cube  corner  and  at  the  center  of  each  cube  face. 
All  the  atoms  in  this  arrangement,  both  corner  and  face  atoms,  are  similarly  situated 
as  regards  symmetry  and  relation  to  neighbors.  Each  atom  is  surrounded  by  twelve 
others,  all  equidistant  and  exactly  similarly  situated  for  every  atom.  It  is  this  high 
degree  of  symmetry,  combined  with  the  close  packing,  that  makes  substances  of  this  type 
so  ductile.  This  arrangement  is  known  as  "cubic  close-packing "  and  is  one  of  the  two 
alternative  arrangements  that  equal  hard  spheres  assume  (Fig.  Ib)  when  pressed  tightly 


215 


together,  with  sufficient  shaking  to  allow  them  to  find  their  places.  This  suggests,  and 
the  other  evidence  at  hand  points  to  the  same  conclusion,1  that  the  atoms  of  the  sub- 
stances which  have  this  arrangement  are  fairly  spherical  in  shape.  The  necessary 
shaking  corresponds  to  the  temperature  required  for  ' '  annealing. ' '  The  only  difference 
between  the  packing  of  balls  and  that  of  atoms  of  this  kind  is  the  ability  of  the  atoms  to 
hold  on  to  each  other  after  they  have  found  their  places. 

There  is  one  important  exception  to  the  rule  that  the  most  ductile,  and,  therefore, 
the  most  generally  useful  substances  are  those  whose  atoms  are  in  face-centered  cubic 
arrangement,  viz.,  iron.  The  atoms  of  iron,  and  also  of  chronium, molybdenum,  tungsten, 
and  the  alkali  metals,  are  in  centered  cubic  arrangement  (Fig.  2).  This  arrangement  is 
obtained  by  dividing  the  space  occupied  by  a  single  crystal  or  grain  into  equal  close- 
packed  cubes,  and  placing  an  atom  at  each  cube  corner  and  each  cube  center.  The  two 
sets  of  atoms,  the  "corner-atoms"  and  the  "center  atoms,"  are  interchangeable,  so 
that  if  the  system  of  lines  in  Fig.  2  had  started  with  one  of  the  center  atoms,  all  the 
corner  atoms  in  Fig.  2  would  become  center  atoms  and  vice  versa.  Each  atom,  wrhether 
center  or  corner  atom,  is  surrounded  by  eight  others  in  perfect  cubic  arrangement  about 
it,  situated  alwavs  in  the  same  direction  and  at  the  same  distances.  Hence  this 


Fig.  2.     Centered  Cubic  Arrangement 

arrangement  has  the  same  high  degree  of  symmetry  as  the  face-centered  cubic  arrange- 
ment. It  is  not,  however,  as  closely  packed.  Smooth,  hard  spheres  cannot  be  packed 
in  centered  cubic  arrangement  except  by  the  use  of  constraints,  and  when  so  packed 
are  in  unstable  equilibrium.  A  slight  jar  causes  them  to  reassemble  in  one  of  the  close- 
packed  arrangements.  (Figs.  1  and  3).  It  is  evident,  then,  that  the  atoms  of  these  ele- 
ments are  either  not  spherical  or  that  they  possess  some  special  forces  of  attraction 
localized  at  cube  corners  (see  Fig.  15). 

The  third  type  of  arrangement  found  in  metals  is  the  hexagonal  close-packed  arrange- 
ment (Fig.  3).  It  is  the  second  of  the  two  alternative  arrangements  which  equal  hard 
spheres  assume  when  closely  packed  by  pressure  and  shaking.  It  is  less  symmetrical 
than  the  cubic  close  packed  arrangement,  but  equally  close  packed.  The  two  are 
closely  related,  and  each  can  be  produced  from  the  other  by  a  simple  gliding,  as  will  be 
shown  later.  This  is  the  arrangement  taken  by  the  atoms  of  magnesium,  zinc,  cad- 
mium electrolytic  cobalt,  and  probably  to  some  extent  by  all  cubic  close-packed  metals 
when  strained  (so  as  to  cause  gliding.)  This  arrangement  is  formed  by  dividing  the 
space  occupied  by  a  single  crystal  of  the  substance  into  a  series  of  equal  closely  packed 
right  triangular  prisms,  the  bases  of  which  are  equilateral  triangles,  and  the  altitudes 
equal  to  1.633  times  the  length  of  the  sides  of  the  triangles.  (Fig.  3.)  An  atom  is 

1  cf.  Langmuir,  1.  c.  p.  878. 


216 


Fig.  3.     Hexagonal  Close  Packed  Arrangement 


Fig.  5.     Tetrahedral  Arrangement 


21' 


located  at  each  prism  corner  and  at  half  of  the  prism  centers.  This  arrangement  is 
simpler  and  more  symmetrical  than  it  appears.  Each  atom  is  surrounded  by  twelve 
others,  all  equidistant  and  uniformly  spaced  about  it  in  dodecahedral  arrangement. 
These  dodecahedra  are  of  exactly  the  same  dimensions  as  in  the  cubic  close-packed  ar- 
rangement, but  are  not  quite  regular,  the  upper  half  being  rotated  60  deg.  from  the  posi- 
tion corresponding  to  a  regular  dodecahedron. 

There  are  two  other  simple  types  of  arrangement,  which,  though  they  do  not  occui 
among  metals,  are  important  for  comparison,  and  their  description  will  make  clearer 
the  distinguishing  features  of  metals. 

The  first  is  the  simple  cubic  arrangement  (Fig.  4) .  It  is  formed  by  dividing  the  space 
occupied  by  a  crystal  into  a  series  of  equal,  closely-packed  cubes,  and  placing  an  atom 
at  each  cube  corner.  All  the  atoms  are  similarly  situated,  each  being  surrounded  by  six 
others,  all  equidistant,  in  the  direction  of  the  cube  faces.  In  spite  of  its  great  simpli- 
city, it  has  only  the  same  degree  of  symmetry  as  the  two  cubic  arrangements  already 
described.  It  is  an  extremely  "loose  packed "  arrangement  for  hard  spheres,  and  would 


Fig.  4.     Simple  Cubic  Arrangement 

be  very  unstable.  A  system  of  equal  cubes,  however,  if  packed  in  this  manner  would 
fill  all  space. 

No  elementary  solid  has  yet  been  found  whose  atoms  arrange  themselves  in  this 
manner.  This  fact  might  be  interpreted  as  evidence  that  none  of  the  atoms  are  cubical 
in  shape.  There  is  strong  evidence,  however,  that  many  of  the  atoms,  especially  those 
of  low  atomic  weight,  are  approximately  cubical  in  shape.1  The  fact  that  when  these 
substances  crystallize,  their  atoms  do  not  pack  together  in  simple  cubic  arrangement, 
is  due  rather  to  the  nature  of  the  forces  holding  them  together  (see  discussion  of  iron, 
tungsten,  etc.,  above).  It  may  be  taken  as  evidence  that  these  forces,  in  the  case  of 
cubical  atoms,  are  not  localized  at  the  centers  of  cube  faces,  but  at  cube  corners. 

The  substances  which  have  this  simple  cubic  arrangement  of  atoms  are  composed 
of  equal  numbers  of  positive  and  negative  ions.  The  positive  and  negative  ions  al- 
ternate in  every  direction  as  shown  in  Figs.  4  and  16,  so  that  each  positive  ion  is  com- 
pletely surrounded  by  six  negative  ions  and  vice  versa.  The  forces  holding  these  atoms 
together  are  different  from  any  of  those  thus  far  considered.  In  the  cases  described 
above,  and  in  the  great  majority  of  compounds,  the  cohesion  is  due  to  the  "stray  fields" 


1  (See  Langmuir,  1.  c..  p.  892  ff.) 


218 


of  the  atoms.  In  these  ion  compounds  it  is  due  to  the  electrostatic  attraction  between 
the  oppositely  charged  ions.  This  is  stronger  than  the  stray  fields,  and  causes  the 
atoms  to  pack  together  as  closely  as  their  shape  will  allow.  The  fact  that  they  choose 
to  pack  in  simple  cubic  arrangement  is  additional  evidence  that  they  are  cubic  in  shape. 
No  ion  compounds  of  this  kind  (i.e.  containing  equal  numbers  of  positive  and  negative 
ions  of  approximately  the  same  size)  between  spherical  atoms  have  yet  been  examined, 
but  it  is  to  be  expected  that  they  will  show  one  of  the  "close  packed"  arrangements 
(face-centered  cubic  or  hexagonal)  described  above. 

The  fifth  simple  type  of  atomic  arrangement  is  the  tetrahedral  arrangement.  (Fig. 
5.)  Each  atom  is  surrounded  by  four  others,  arranged  in  a  regular  tetrahedron  about  it. 
It  is  not  as  symmetrical  as  the  three  cubic  arrangements  described  above  (Figs.  1,  2  and 
4)  for  while  each  atom  is  at  the  center  of  a  tetrahedron  of  neighboring  atoms,  half  of 
these  tetrahedra  are  positive  and  half  negative;  i.e.,  upside  down  with  respect  to  the 
first. 

The  only  substances  thus  far  found  with  the  tetrahedral  arrangement  of  atoms  are 
diamond,  silicon,  and  the  "ion  compound "  NH4  Cl.  There  is  strong  chemical  evidence 
that  in  each  of  these  substances  the  unit  of  structure  (viz.,  the  C  and  Si  atom,  and  the 
NH4  ion)  is  really  tetrahedral  in  shape. 

The  Measuring  Machine.  The  determination  of  these  atomic  arrangements  re- 
requires  the  measurement,  in  as  many  different  directions  as  possible,  of  the  distance 
between  consecutive  planes  of  atoms.  The  arrangement  of  atoms,  whatever  it  may  be, 
is  assumed  to  be  a  regular  one  which  repeats  itself  throughout  the  crystal.1  This 
assumption  can  be  checked  by  the  result.  Through  such  an  arrangement  a  system  of 
equidistant  parallel  planes  can  be  drawn  in  any  direction  whatever  so  as  to  pass  through 
all  the  atoms.  In  most  directions,  these  planes  will  be  very  close  together  and  sparsely 
settled  with  atoms.  In  a  few  particular  directions,  however,  they  will  be  far  apart 
and  densely  populated.  These  are  the  directions  of  easy  cleavage  and  gliding.  It  is 
these  densely  populated  planes  whose  distances  apart  are  measured. 

The  original  measuring  machine,  by  which  the  pioneer  measurements  were  made, 
was  a  special  form  of  ' '  spectrometer. ' '  It  has  been  simply  and  charmingly  described  by 
its  inventors  in  a  book2  worth  reading.  The  measurements  described  in  this  paper  were 
made  with  a  modified  form  of  Bragg  machine.  The  original  machine  was  applicable 
only  to  large,  perfect  crystals,  required  careful  manipulation,  and  was  subject  to  serious 
error  unless  the  crystals  were  very  perfect  and  the  number  of  observations  large.  The 
author's  modification  is  free  from  these  errors,  requires  but  one  simple  observation,  and 
is  applicable  to  all  substances  which  are  crystalline,  i.e.,  all  in  which  there  is  any 
arrangement  to  measure. 

The  complete  machine  is  shown  in  Fig.  6.  It  consists  of  a  small  transformer  (or 
other  source  of  high  potential)  capable  of  supplying  1  kw.  at  about  30,000  peak  volts; 
a  Coolidge  X-ray  tube,  X;  a  thin  sheet  of  properly  chosen  material,  /,  serving  as  filter;  a 
pair  of  slits,  s\  and  sz,  in  metal  sheets,  to  limit  the  beam  of  X-rays;  a  tiny  glass  tube,  T, 
containing  the  powdered  substance  to  be  measured ;  and  a  photographic  plate  or  strip 
of  film  bent  in  arc  of  circle,  F.  The  operation  consists  in  filling  the  glass  tube  with  a 
few  milligrams  of  the  substance  to  be  analyzed,  powdered  as  finely  as  possible ; ' '  loading ' ' 
the  photographic  film  holder;  exposing  over  night  to  X-rays  at  30,000  peak  volts  and  as 
many  milliamperes  as  the  tube  will  carry  safely  without  watching,  (a  maximum  ex- 
posure of  300  milliampere  hours) ;  and  developing  the  film. 


1  If  the  arrangement  is  not  regular,  there  is  nothing  to  be  measured.    Such  "amorphous"  solids  are  very  few  in  num- 
ber, much  fewer  than  was  previously  believed. 

-  W.  H.  and  W.  L.  Bragg,  "X-rays,  and  Crystal  Structure"  G.  Bell  &  Sons,  London. 

219 


Typical  photographs  are  shown  in  Fig.  7a,  and  7b.  Fig.  7a  is  a  photograph 
of  aluminium  filings,  taken  with  a  plate  and  very  short  slits,  so  that  the  trace  of  the 
direct  beam  in  the  center  of  the  plate  is  a  circular  spot.  Fig.  7b  is  a  photograph  of 
molybdenum  powder,  taken  with  circular  film  and  slits  as  shown  in  Fig.  6.1  In  this  case 
the  traces  of  the  circles  on  the  film  show  as  nearly  straight  lines.  The  circles  and  lines 
are  due  to  the  "reflection"  of  the  X-rays  by  the  tiny  crystals  of  molybdenum  in  the 
tube,  as  will  be  described  later.  The  distances  of  these  circles  or  lines  from  the  central 
line  on  the  film  are  nearly  proportional,  inversely,  to  the  distances  between  the  planes  of 
atoms,  and  from  them  these  atomic  distances  can  be  easily  and  quickly  calculated. 
Some  examples  of  calculation  are  given  below. 

The  Measuring  Rod.  The  measuring  rod  by  which  these  atomic  distances  are 
measured  is  the  wave-length  of  a  particular  X-ray. 

The  possibility  of  measuring  the  dimensions  of  any  physical  body  depends,  primarily, 
upon  the  possession  of  a  measuring  rod  of  length  comparable  with  the  dimensions  to  be 
measured.  Thus,  the  discovery  and  calibration  of  wave-lengths  of  visible  light  opened 


Fig.  6.     Powder  Photograph  Apparatus 

up  a  whole  new  field  of  measurements,  comparable  in  length  with  the  new  measuring 
rod,  such  as  the  thickness  of  films,  imperfection  of  polished  surfaces,  displacement  of 
vibrating  membrances,  increase  in  length  due  to  thermal  expansion,  etc.  In  the  same 
way,  the  discovery  that  X-rays  are  of  the  same  nature  as  light,  and  the  isolation  and 
calibration  of  X-ray  wave-lengths,  opened  up  a  vast  new  field  of  measurements  of 
dimensions  comparable  with  the  wave-length  of  X-rays,  viz.,  atomic  dimensions. 

We  are  accustomed  to  think  of  the  measurement  of  things  too  large  or  too  small  to 
see  and  touch  as  necessarily  very  rough  and  approximate.  It  is  somewhat  of  a  surprise, 
therefore,  to  note  that  the  only  measurements  accurate  enough  to  justify  the  use  of 
eight-place  logarithm  tables  are  those  of  astronomy;  that  wave-lengths  of  light  are 
measured  to  1  part  in  10,000,000;  and  that  the  wave-length  of  X-rays,  and  by  means  of 
it,  the  distances  between  atoms,  can  easily  be  measured  to  1  part  in  100,000. 

The  spectrum  of  X-rays  is  exactly  like  that  of  visible  light,  except  that  the  wave- 
lengths are  shorter.  It  consists  (Fig.  8)  of  bright  lines  superimposed  upon  a  continuous 
spectrum.  The  wave-lengths  in  the  X-ray  spectrum  depend  upon  anode  material  and 
voltage  (Figs.  9  and  10)  in  exactly  the  same  way  that  the  wave  lengths  in  the  visible 
spectrum  depend  upon  incandescent  material  and  temperature.  And  just  as  it  is 

1  The  central  line  and  first  "reflected"  line  in  Fig.  7b  have  been  partially  absorbed  by  a  "stepped"  filter,  placed 
directly  in  front  of  the  film,  for  the  purpose  of  measuring  the  intensity  of  the  lines. 


220 


Fig.  7a.     Aluminum 


Fig.  8.     X-ray  Spectrum  of  Tungsten 


221 


possible  to  obtain  nearly  monochromatic  yellow  light  by  putting  salt  in  a  flame  under 
proper  conditions,  so  by  running  an  X-ray  tube  with  proper  anode  at  the  right  voltage, 
it  is  possible  to  produce  a  single  wave  length  (line)  of  such  great  intensity  that 
practically  all  the  rest  of  the  spectrum  can  be  absorbed  by  a  properly  chosen  filter, 
leaving  nearly  monochromatic  X-rays.1  (Fig.  10.)  It  is  in  this  way  that  the  mono- 
chromatic X-rays  used  in  these  measurements  are  produced.  The  measurements 
described  in  this  paper  were  made  with  X-rays  from  a  molybdenum  target  operated  at 
23,000  volts  constant  potential,  and  the  filter  was  powdered  crystai  zircon,  pressed, 


100 


NO.    Cl    Cf9YSTKL- 
A/0 

- I "  QO  KV 

CURVGNO.  S=8OKV. 


Fig.  9.     Effect  of  Voltage  on  X-ray  Spectrum  of  Tungsten 

with  a  small  amount  of  organic  binder,  into  a  sheet  %  mm.  thick.    The  resulting 
spectrum,  before  and  after  filtering,  is  shown  in  Fig.  10. 

Calibration  of  Measuring  Rod.  The  measurement  of  the  wave-length  of  X-rays 
in  centimeters  was  part  of  the  pioneer  work  of  the  Braggs,2  and  was  accomplished 
in  the  same  way  as  the  measurement  of  visible  wave-lengths,  viz.  by  the  use  of  a 
"grating"  of  known  dimensions.  It  is  interlocked  with  the  determination  of  crystal 
structure,  since  the  grating  used  was  a  crystal,  and  it  was  necessary,  before  using  it,  to 
determine  its  dimensions,  i.e.  the  arrangement  of  atoms  in  it.  The  procedure  was  that 
of  experiment  and  trial.  Preliminary  experiments  indicated  that  the  atoms  of  rock 
salt  were  in  simple  cubic  arrangement,  as  shown  in  Fig.  4.  Assuming  this  to  be  so, 


1  For  a  more  detailed  description,  see  Hull,  Phys.,  Rev.  10, 

2  X-rays  and  Crystal  Structure,  p.  110. 


5,  1917. 


222 


a  rock  salt  crystal  was  used  as  a  grating  to  measure  tentatively  the  wave-lengths  of 
X-rays.  These  wave-lengths  were  then  used  for  further  investigation  of  the  arrange- 
ment of  atoms  in  rock  salt  and  other  crystals,  and  were  checked  and  corrected  by 
successive  trials. 

The  method  of  using  the  rock-salt  grating  is  shown  in  Fig.  11.  The  crystal  C, 
with  its  planes  of  atoms  perpendicular  to  the  paper,  is  placed  in  the  path  of  a  narrow 
beam  of  X-rays.  Each  plane  of  atoms  acts  like  a  mirror  and  reflects  a  small  fraction 
of  the  rays.  The  reflection  is  a  maximum  when  the  reflected  waves  from  all  the  planes 
(many  million,  except  in  the  case  of  very  long  wave-lengths)  are  in  phase,  that  is, 
when  each  is  an  exact  wave-length  or  some  whole  number  of  wave-lengths  behind 
the  next.  It  is  easy  to  show  that  this  is  true  only  when 

n  X  =  2  d  sin  9  (1) 

where  n  is  an  integer,  usually  1  or  2,  X  the  wave-length,  d  the  distance  between  planes  of 
atoms,  and  6  the  angle  of  incidence  (i  in  Fig.  11)  of  the  rays  on  the  crystal.1 


Spectrum  of  MO  X  Rays 
at  28,000  Volts 


0.35  MM  Zr  Si 04  Filter 
0.58MMZrSi04Fi!tei 


0.46    0.50     0,54    0.58    0.62     0.66     0.70     0.74    0.78     0.82    0.86 
Fig.  10.     Effect  of  Filtering  on  Molybdenum  X-ray  Spectrum 

The  determination  of  X  requires,  therefore,  the  knowledge  of  n,  d,  and  6.  6  can  be 
observed,  and  the  orders,  n,  counted,  d  must  be  determined  initially  from  the  physical 
properties  of  the  crystal,  viz.  its  density,  atomic  weights,  and  arrangement  of  atoms. 
This  can  best  be  explained  by  an  example : 

The  atoms  of  rock  salt  are  in  simple  cubic  arrangement,  as  shown  in  Fig.  4.  There 
is  one  atom  to  each  cube,  as  can  be  seen  by  displacing  all  the  atoms  in  Fig.  4  in  the  di- 
rection of  a  cube  diagonal  to  the  cube  centers.  Half  of  the  cubes  contain  sodium  atoms 
and  half  chlorine  atoms.  The  average  weight  per  cube  is  the  mean  of  the  weights  of  one 
sodium  and  one  chlorine  atom,  viz.  %  (38.00  X  l(T24g-f- 58.50 Xl(T24g)  =48.25  X10~24g. 

48  ^5  X 1 0~24 
Hence  the  density,  which  is  the  weight  per  unit  volume,  must  be  equal  to  — 

dA 

where  d  is  the  side  of  one  of  the  small  cubes  (Fig.  4,)  i.e.,  the  distance  between  planes  of 
atoms  parallel  to  the  cube  faces.  This  value  of  density  must  be  the  same  as  that 
obtained  by  measuring  and  weighing  a  large  crystal,  viz.  2.174.  This  gives 


-'4 


48.25  X10"24 
2.174 


=  2.814  Xl(T8cm.  =  2.814A2 


1  It  will  be  observed  that  in  order  to  measure  different  wave-lengths  the  crystal  must  be  rotated.  This  is  the  only 
essential  difference  between  the  crystal  grating  and  the  ordinary  diffraction  grating. 

?  The  Angstrom  unit  or  A  (  =10"*  cm.)  which  is  the  standard  unit  for  expressing  wave-lengths  of  visible  light,  is  even 
better  suited  to  atomic  and  X-ray  measurements.  It  will  be  used  or  assumed  in  all  the  following  discussions. 


223 


The  wave-length  of  the  "a  doublet"  of  molybdenum  determined  in  this  way  is 
0.712  A.1  This  is  the  "measuring  rod"  with  which  the  following  measurements  were 
made. 

Interpretation  of  Powder  Photographs.  The  X-ray  wave  length  thus  calibrated  can 
now  be  used  to  measure  atomic  distances.  If,  in  the  arrangement  shown  in  Fig.  11,  the 
-X-rays  are  made  monochromatic  by  proper  voltage  and  filtering,  then  as  the  crystal  is 
rotated  a  series  of  intense  reflections  will  be  observed  at  angles  whose  sines  are  in  the 
ratio  1:2:3,  etc.,  corresponding  to  successive  integral  values  of  n  in  Eq.  1.  If  a  new  face 
is  ground  on  the  crystal  at  an  angle  to  the  first,  and  exposed  to  the  rays  in  the  same  way, 
another  similar  series  of  reflections  will  be  observed,  at  different  angles,  corresponding 
to  the  different  distance  (d,  Eq.  1)  between  the  planes  parallel  to  this  new  face.  The 
process  of  analyzing  a  crystal  consists  in  observing  these  reflections  from  as  many  faces 
as  possible,  and  calculating,  from  Eq.  1,  the  distance  between  the  planes  parallel  to 
them.  When  a  single  crystal  is  used  this  requires  many  observations.  The  work  is 
greatly  simplified  by  using  a  powder,  in  which  all  possible  orientations  are  represented 


Fig.  11.     X-ray  Spectrometer 

at  the  same  time  by  one  or  more  of  the  tiny  crystals,  and  photographing  simultaneously 
the  reflections  from  all  these  little  crystals.  This  is  the  method  sketched  in  Fig.  G,  and 
gives  patterns  of  lines  like  Figs.  7,  12  and  13.  It  might  be  expected  that  the  number  of 
lines  in  these  patterns  would  be  infinite,  since  there  is  an  infinite  number  of  different 
possible  planes  in  any  crystal.  The  reflections  from  most  of  the  planes,  however, 
come  at  angles  whose  sines  are  greater  than  1  (as  is  evident  from  equation  [l]  when  d 
is  small)  and  which,  therefore,  do  not  exist.2 

The  analysis  of  these  photographs  is  very  simple  in  the  case  of  simple  substances, 
like  pure  metals.  It  consists  in  finding,  by  successive  trials,  an  arrangement  of  atoms 
whose  planar  spacings,  beginning  with  the  planes  farthest  apart  and  skipping  none, 
exactly  fit  the  observed  pattern  of  lines.  The  calculation  of  the  planar  spacings  for  all 
the  important  planes  is  not  difficult,  and  with  simple  substances  but  few  trials  are 
necessary. 

1  The  best  measurements  of  X-ray  wave-lengths  are  those  of  Siegbahn.  Verh.  Deut.  Phys.  Ges.  13,300,  1917. 

2  This  means  physically  that  there  is  no  angle  of  incidence  large  enough  to  make  the  reflection  from  one  plane  a  whole 
wave-length  behind  that  from  the  one  in  front. 

224 


Fig.  12.     Typical  X-ray  Powder  Photographs  of  Metals 


Fig.  13.     Typical  X-ray  Powder  Photographs  of  Salts 


225 


The  method  of  calculation  can  best  be  shown  by  an  example.  Potassium  chloride 
(Fig.  13)  gives  the  simplest  pattern  of  lines  yet  observed.  It  consists  of  6  regularly 
spaced  lines,  then  a  slight  gap,  then  7  more  lines,  (these  may  not  all  show  in  the  repro- 
duction), then  another  gap,  etc.  The  best  first  guess  is  that  a  simple  pattern  like  this 
corresponds  to  a  simple  cubic  arrangement.  The  equation  of  solid  geometry  for  the 
distance  between  planes  in  a  simple  cubic  arrangement  is 

do 

d  =  ^=  (2) 

\//*2+£2+/2 

where  h,  k,  and  I  are  any  whole  numbers,  and  d0  is  the  side  of  the  cube.  The  numbers 
/z,  k,  I,  are  called  the  "indices  "  of  the  planes.  They  are  the  reciprocals  of  the  intercepts 
of  the  planes  on  the  X,  Y  and  Z  axes  respectively. 

do 

The  largest  distance  is  ~~,—,  corresponding  to  the  "indices"  1,  0,  0.    This  gives  the 

VI 

<4 

line  nearest  the  center  (see  equation  1).     The  next  line  is  ~=s    given   by   the   planes 


whose  indices  are  1,  1,  0.     Then  follow  -7=  (1,  1,  1),  ~~-(2,  0,  0),  -  ~=-  (2,  1,  0),  and 

V3  V4  V5 

do  do 

.—  (2,  1,1).     The  next  line,  corresponding  to     ,—  ,  should  be  lacking,  as  there  are 
V6  V  7 

no  three  numbers  the  sum  of  whose  squares  is  7.     Fig.  13  shows  that  it  is  actually 

lacking.     Then  follow,  in  regular  order,  —j=--  (2,  2,  0),  —  £=  (2,  2,  1  and  3,  0,  0),  •-,=  (3, 

vs  v9  yip 

1    0)   -  -^L  (3,  lf  1),  --^L  (2,  2,  2),  TT=>  (3,  2,  0)  and  77==  (3,  2,  1).    Then  should  come 

Vn  Vi2  vis  yii 

do 

another  gap,  corresponding  to  "7^-,  which  cannot  be  formed  from  the  squares  of  three 

\  15 

integers.  These  regularities  can  be  better  seen  in  some  of  the  other  patterns  in  Fig.  13, 
which  are  exactly  similar  to  that  of  KC1,  except  that  there  are  extra  lines  just  in  front  of 
the  first,  third  and  fifth  regular  lines,  due  to  the  difference  in  weight  and  size  of  the  two 
atoms  of  which  these  salts  are  composed.  The  calculation  of  the  positions  of  these 
extra  lines  is  also  simple,  but  will  not  be  given  here. 

Returning  to  KC1,  the  positions  of  the  lines  calculated  from  the  above  series  of 
inverse  square  roots  are  found  to  coincide  exactly  with  the  observed  pattern.  This 
proves  that  the  first  guess  was  correct,  that  the  atoms  of  KC1  are  in  simple  cubic 
arrangement.  The  result  can  be  independently  checked,  however,  by  the  density.  For 
the  average  mass  in  each  of  the  small  cubes,  divided  by  the  volume  of  the  cube,  must 
equal  the  known  density  of  the  substance.  The  value  of  d0  calculated  from  equation  1 
is  3.13A.  The  average  weight  of  potassium  and  chlorine  atoms  is  }/i  (64.5+58.5)  X  1(T24 

C>1  5  X  10  24 
g  =  61.5X10~24  g.     This  gives  for  the  density  -r  —          —  -  =2.01  which  checks  with 

the  standard  value  within  experimental  error1. 

One  more  example  may  be  briefly  cited,  viz.  molybdenum.  Its  pattern  is  shown  in 
Fig.  7b.  It  is  found  that  the  series  of  lines  which  corresponds  to  a  simple  cubic 
arrangement  does  not  fit  this  pattern.  We  therefore  try  a  centered  cubic  arrangement. 

1   These  X-ray  determinations  of  density  are,  in  general,  much  more  accurate  than  the  standard  ones. 

226 


The  distance  between  planes  in  this  arrangement  is  calculated  in  the  same  way  as  for 
the  simple  cubic  arrangement,  except  that  half  the  lines,  viz.  those  due  to  planes  the 
sum  of  whose  indices  are  odd,  are  lacking1.  Hence  the  relative  spacings,  beginning  with 

1        1        1 
the  largest,  should  be  proportional  to  ~~F,  — 7=^  — /=,  etc.    The  series  of  lines  calculated 


V2   V4 

from  these  values  is  found  to  fit  the  pattern  of  Fig.  10  exactly.    The  length  do  of  the  side 
of  the  small  cube  is  3.  loA.    As  check,  the  density  of  molybdenum  should  be  equal  to  the 

318  XI  0~24 
weight  of  two  molybdenum  atoms  divided  by  the  volume  of  the  cube,  i.e.  -- 


(3.15X10"8)3 


=  10.0  which  is  the  correct  value. 


DISCUSSION  OF  RESULTS 

The  results  of  the  measurements  that  have  been  made  thus  far  are  summarized  in 
the  following  table.    The  list  includes  most  of  the  metals  that  are  easily  obtainable  in 


Substance 

Arrangement  of  Atoms 

Length  of  side  of 
Elementary  Cube 
In  Angstroms 

Distance  between 
Nearest  Atoms 
(Between  Centers)  In 
Angstroms 

Aluminum 

Face-Centered  Cubic 

4.05 

2.86 

Cobalt  

(Cubic  close  packed) 

3.57 

2.52 

Nickel         

(Cubic  close  packed) 

3.54 

2.50 

Copper                      

(Cubic  close  packed) 

3.60 

2.54 

Rhodium  

(Cubic  close  packed) 

3.82 

2.70 

Silver  

(Cubic  close  packed) 

4.06 

2.87 

Platinum    .  .        

(Cubic  close  packed) 

4.02 

2.85 

Gold  

(Cubic  close  packed) 

4.08 

2.88 

Lead  

(Cubic  close  packed) 

4.92 

3.48 

Lithium    

Centered  Cubic 

3.50 

3.03 

Sodium  

Centered  Cubic 

4.30 

3.72 

Chromium  

Centered  Cubic 

2.91 

2.52 

Iron         

Centered  Cubic 

2.86 

2.48 

Molybdenum  

Centered  Cubic 

3.15 

2.73 

Tunsten  

Centered  Cubic 

3.15 

2.73 

Magnesium  

Hexagonal  (close  packed) 

3.22 

Zinc       

Hexagonal  (close  packed) 

2.84 

Cadmium     

Hexagonal  (close  packed) 

3.15 

Cobalt  

Hexagonal  (close  packed) 

2.53 

Diamond  

Tetrahedral 

3.56 

1.54 

Silicon            

Tetrahedral 

5.43 

2.35 

Lithuim  Fluoride 
Sodium  Fluoride  

Simple  Cubic 
Simple  Cubic 

2.01 
2.31 

2.01 
2.31 

Sodium  Chloride  

Simple  Cubic 

2.81 

2.81 

Potassium  Fluoride  

Simple  Cubic 

2.69 

2.69 

Potassium  Chloride  

Simple  Cubic 

3.13 

3.13 

Potassium  Iodide  

Simple  Cubic 

3.55 

3.55 

Magnesium  Oxide  

Simple  Cubic 

2.11 

2.11 

pure  form,  a  few  simple  salts,  as  examples,  and  the  non-metallic  elements  carbon  (dia- 
mond) and  silicon.  The  structure  of  graphite  is  less  simple,  and  will  not  be  discussed 
here.  It  apparently  contains  a  mixture  of  forms,  produced  by  gliding,  like  cobalt. 

The  case  of  cobalt  is  exceptional.  A  finely  powdered  sample  produced  by  rapid 
electrolysis  showed  a  mixture  of  cubic  and  hexagonal  close-packing  in  nearly  equal  ratio. 
After  annealing  in  hydrogen  at  600  deg.  this  sample  showed  only  the  cubic  form. 
Another  sample,  composed  of  filings  from  pure  cast  metal,  showed  slight  traces  of 
hexagonal  packing,  due  presumably  to  straining.  It  is  probable  that  the  other  cubic 
close-packed  metals  will  behave  in  a  similar  manner,  but  this  question  has  not  been 
studied. 


For  details  and  theory  see  Hull,  Phys.  Ret:  10,67.3,  1917. 


227 


PHYSICAL  PROPERTIES 

Many  of  the  physical  properties  of  these  substances  are  evident  from  an  inspection 
of  their  atomic  models.  Thus  it  is  clear  why  metals  with  face-centered  cubic  arrange- 
ment are  soft  and  ductile.  For  in  the  planes  parallel  to  the  octahedral  faces  (the 
bevelled  face  in  Fig.  Ic)  the  atoms  are  so  closely  packed  that  one  plane  can  slip  over 
the  one  beneath  it  without  appreciable  elevation,  and  without  the  atoms  getting  away 
from  each  other's  attractive  influence.  There  are  4  sets  of  planes  parallel  to  which  this 
easy  gliding  can  take  place,  liz.  those  parallel  to  the  4  pairs  of  octahedral  faces,  and  in 
each  of  these  planes  there  are  three  equally  easy  directions.  There  is  no  direction, 
therefore,  in  which  a  shearing  force  can  be  applied  to  a  crystal  of  this  kind  which  would 
-not  be  nearly  parallel  to  one  of  these  directions  of  easy  gliding. 


Fig.  14a.     Arrangement  of  Atoms  in  Successive  Planes  in  Cubic  Close  Packing 


If  the  shear  takes  place  in  such  uniform  manner  that  each  plane  moves  over  the  one 
beneath  it  by  the  same  amount,  the  original  arrangement  is  reproduced  and  no  strain 
results.  If  only  the  alternate  planes  move,  however,  the  arrangement  becomes  hexa- 
gonal close  packing.  This  is  evident  from  Figs.  14a  and  14b,  which  give  the  arrange- 
ment of  atoms  in  successive  close-packed  planes  (dotted  and  heavy  lines  respectively) 
in  the  cubic  and  hexagonal  arrangements  respectively.  In  the  cubic  arrangement  there 
are  three  different  positions  of  the  planes,  and  these  alternate  1,  2,  3,  1,  2,  3,  etc.  In  the 
hexagonal  arrangment  there  are  only  two  positions,  which  alternate,  1,2,  1,2,  etc. 

In  the  hexagonal  close-packed  arrangement  there  is  only  one  set  of  planes  parallel 
to  which  this  easy  gliding  can  take  place.  Hence  a  strained  metal  becomes  less  ductile 

228 


to  the  extent  to  which  its  arrangement  becomes  hexagonal,  and  it  is  evident  that 
mechanical  working  should  produce  hardening. 

The  centered  cubic  metals  should  be  somewhat  less  ductile  than  the  face-centered, 
for  the  atoms  hold  on  to  each  other  at  only  8  points  (Fig.  15)  instead  of  12,  and  are 
more  likely  to  move  out  of  each  other's  influence  during  gliding. 

In  the  tetrahedral  arrangement  gliding  is  impossible.  The  atoms  touch  at  only  4 
points,  and  in  jumping  from  one  stable  position  to  the  next  would  entirely  lose  hold 
of  each  other.  This  firm  interlocking  also  accounts  for  the  hardness  of  these  substances. 

In  the  case  of  the  simple  cubic  salts  (Fig.  16)  gliding  is  impossible  for  a  different 
reason.  The  atoms  are  held  together  by  electrostatic  attraction,  each  ion  being  sur- 
rounded by  six  oppositely  charged  ions.  The  process  of  gliding  would  bring  ions  of  like 
charge  opposite  each  other,  with  resulting  repulsion  and  cleavage. 


Fig.  14b.     Arrangement  of  Atoms  in  Successive  Planes  in  Hexagonal  Close  Packing 

The  electrical  conductivity  of  metals  depends  on  the  ability  of  electrons  to  move 
between  the  atoms.  A  discussion  of  this  without  a  better  knowledge  of  the  shape  and 
size  of  atoms  would  be  premature.  It  can  be  seen  at  once,  however,  why  "ions  salts" 
and  crystals  like  diamond  are  nonconductors.  In  each  of  these  arrangements  the  elec- 
trons in  the  atoms  are  in  complete  groups  of  eight,  which  is  such  a  stable  arrangement1 
that  large  forces  (corresponding  to  the  dielectric  strength  of  the  substance)  are  re- 
quired to  remove  them.  The  atoms  of  metals,  on  the  other  hand,  have  extra  electrons 
which  cannot  find  places  in  these  stable  shells,  and  are  therefore  "free"  to  move  from 
atom  to  atom. 


1   Langmuir,  1.  c.  p.  873. 


MAGNETIC  PROPERTIES 

It  is  well  known  that  the  ferro-magnetism  of  iron  is  not  a  specific  property  of  the 
iron  atom,  since  iron  in  solution  and  in  compounds  is  in  general  not  ferro-magnetic. 
The  ferro-magnetism  must  depend,  not  only  on  the  nature  of  the  atoms,  but  on  the  way 
in  which  they  are  grouped  together.  It  might  have  been  anticipated,  therefore,  that 


Fig.  15.     Packing  of  Atoms  in  Centered  Cubic  Arrangement 

the  cause  of  ferro-magnetism  was  the  centered  cubic  arrangement  which  is  characteristic 
of  iron.  A  glance  at  Table  I  shows  that  this  is  not  the  case.  Nickel,  which  is  ferro- 
magnetic, has  a  face-centered  cubic  arrangement,  like  copper.  Cobalt  is  sometimes 
like  copper,  sometimes  like  magnesium.  Neither  is  like  iron.  Chromium,  on  the  other 


Fig.  16.     Packing  of  Atoms  in  Simple  Cubic  Arrangement 

hand,  which  is  not  ferro-magnetic,  has  a  centered  cubic  arrangement  like  iron.  Man- 
ganese has  not  yet  been  obtained  sufficiently  pure  to  determine  its  arrangement.  It  is 
evident,  therefore,  that  while  the  centered  cubic  arrangement  may  be  favorable  to 
ferro-magnetism,  and  may  make  iron  more  magnetic  than  cobalt  and  nickel,  it  is  not 
the  principal  or  even  an  essential  factor. 


SOME  INTERESTING  APPLICATIONS  OF  THE  COOLIDGE  X-RAY  TUBE 

BY  DR.   WHEELER  P.   DAVEY 

There  have  lately  been  published  many  very  interesting  radiographs  of  flowers, 
leaves  and  insects.  Typical  instances  are  shown  by  Pierre  Goby,  Hall-Edwards  and 
others.  These  men  have  found  it  desirable  in  such  work  to  use  tubes  in  which  the  vac- 
uum was  widely  different  from  that  employed  in  ordinary  radiographic  work.  For  in- 
stance, in  referring  to  a  splendid  radiograph  of  tulip  blossoms,  Hall-Edwards  (Archives 
of  the  Roentgen  Ray,  June,  1914),  who  is  using  tubes  of  the  ordinary  type  (in  this  case 
a  ''  Muller  "  with  a  Bauer  regulator)  notes  that  "it  is  rather  dangerous  to  use  new  tubes 
for  this  purpose  (although  the  best  results  can  undoubtedly  be  obtained  by  them) ,  for  the 
reason  that  it  is  very  easy  to  pass  the  boundary  line  and  get  a  vacuum  so  low  that  it 
cannot  be  raised  without  re-exhausting  the  tube."  Such  difficulties,  though  they  have 
not  prevented  securing  beautiful  radiographs,  have  doubtless  kept  this  method  of  in- 
ternal and  structural  photography  from  advancing  as  rapidly  as  it  might  otherwise  have 
done. 

The  Coolidge  X-Ray  Tube  (see  Physical  Review,  December,  1913,  or  General 
Electric  Review,  February,  1914),  has  proved  itself  to  be  an  efficient  tool  in  the  hand  of 
the  medical  profession,  and  it  occurred  to  the  writer  that  it  would  prove  equally  valu- 
able to  botanists  and  biologists  in  connection  with  the  study  of  plant  and  animal  life. 

The  main  advantages  in  the  use  of  the  Coolidge  tube  are  (1)  the  independence  of  the 
quantity  and  the  penetrating  ability  of  the  rays  produced  (2)  the  ease  and  rapidity  with 
which  the  quantity  and  penetration  of  the  rays  may  be  regulated,  and  (3)  the  fact  that 
when  the  tube  is  once  adjusted  to  the  requirements  of  the  operator  it  needs  no  further 
attention.  To  bring  the  tube  to  any  desired  adjustment,  the  operator  pulls  a  handle 
which  regulates  the  current  through  the  tube,  thus  determining  definitely  the  quantity 
of  X-rays  produced.  He  then  adjusts  the  voltage  across  the  tube  until  the  penetration 
is  of  the  desired  degree.  These  adjustments  are  rapid  and  require  the  minimum  of 
technical  skill. 

To  illustrate  this,  the  writer  took  a  tube  at  random  (it  happened  to  be  No.  280)  from 
the  rack  and  took  a  number  of  radiographs  of  various  botanical  specimens.  It  was  at 
once  found  that,  when  the  proper  penetration  and  exposure  had  been  determined,  the 
radiographs  could  be  duplicated  time  after  time  with  absolute  precision.  Then  biologi- 
cal specimens  were  tried,  and  finally  radiographs  were  taken  of  dense  objects,  such  as 
fuse  plugs  and  Ingersoll  watches.  As  in  the  case  of  the  botanical  specimens,  all  of  these 
radiographs  could  be  accurately  reproduced  as  often  as  desired. 

About  this  time  tube  No.  280  was  accidentally  broken.  Another  tube  (No.  297) 
was  picked  at  random  from  the  rack  and  much  of  the  former  work  was  repeated  with  it . 
There  was  no  difference  noticeable  between  the  two  sets  of  pictures. 

It  was  the  original  intention  to  publish  only  those  radiographs  taken  with  tube  No. 
280,  but  it  was  later  decided  to  add  such  of  those  taken  with  No.  297  as  seemed  to  be  of 
peculiar  interest.  The  radiographs  reproduced  here  were  not  chosen  from  the  total 
number  taken  because  of  any  novelty  or  excellence  in  the  pictures  themselves,  but 
rather  because  they  show  what  a  wide  range  of  work  can  be  done  with  a  single  Coolidge 
tube,  and  because  they  suggest  that  the  Coolidge  tube  is  destined  to  become  a  precision- 
instrument  of  value  to  the  botanist,  biologist  and  mineralogist  as  well  as  to  the  pyhsicist 
and  the  physician. 

None  of  the  pictures  shown  here  has  been  re-touched  or  altered  in  any  way. 

Copyright,  1914,  by  General  Electric  Company. 

231 


Fig.  1.     Fern  Bud  and  Dandelion  Bud.     Tube  280  Fig.  2.     Fern  Leaf  with  Colonies  of  Bugs.     Tube  280 


Fig.  3.     Cherry  Twigs,  with  Young  Leaves,  Buds  and 
Freshly  Opened  Flower.     Tube  280 


Fig.  4.     Cherry  Twigs  with   Mature  Blossoms.     A  Couple 
of  the  Blossoms  have  already  lost  their  Petals.     Tube  28 


232 


I 


Fig   5.     Another  Branch  of  Mature  Cherry  Blossoms 
and  a  Spray  of  Leaves.     Tube  280 


Fig.  7.     Cherry  Twig.     The  Cherry  is  started  to  form 
around  the  Pit.     Tube  280 


Fig.  6.     Cherry  Twig.      All  the  Blossoms  have  lost  their 
Petals.    The  Cherry  Pits  have  started  to  form.    Tube  280 


Fig.  8.     Cherry  Twig.     The  Cherries  are  about  half 
developed.     Tube  297 


233 


Fig.  9.    Golden-Rod  Gall.     The  worm  inside  is  alive.    After 
this  picture  was  taken,  the  worm  finished  its  metamor- 
phosis and  the  resulting  fly  "Eurosta  (Trypeta) 
solidaginis,"   made  its  way  out  into  the 
laboratory.     Tube  280 


Fig.  10. 


Golden-Rod  Galls.     The  Worm  inside  is  dead. 
Tube  280 


Fig.   11.     Crayfish.     This  subject  differs  from  the 

tadpoles,  which  follow,  in  that  the  bony  structure 

is  on  the  outside.     Tube  297 


Fig.  12.     Young  Tadpoles.     Note  the  shape  of  the  intes- 
tines, and  the  beginning  of  ossification  of  the  first  ver- 
tebra.   The  intestines  show  up  strongly  in  contrast 
to  the  surrounding  flesh  because  the  natural 
food  with  which  they  have  gorged  them- 
selves is  rather  opaque  to  the  X-rays. 
Tube  297 


234 


Fig.  15.     Still  Older  Tadpoles.      Note  growth  of  the  intes- 
tines, and  further  ossification  of  vertebra.     The 
neck  is  beginning  to  form.     Tube  297 


Fig.  13.     Still  Older  Tadpoles.     In  the  oldest  of  the  three 
the  vertebra  show  considerable  ossification.     Tube  297 


Fig.  14.    Tadpole,  Size  of  Largest  One  of  Fig.  13.     Magnified 

three  and  one-half  diameters.     Note  articles  of  food 

in  the  intestines.     Tube  280 


Fig.  16.     A  Young  Frog.      The  intestines  are  taking  their 

final  shape.     Ossification  of  bones  is  nearly  complete. 

Tube  297 


Fig.  17.     Partly  Hatched  Egg.     Tube  280 


Fig.  18.     Egg  Nearly  Hatched.     Tube  280 


Fig.  19.    Four-legged  Chicken,  Five  Hours  Old.    Tube  297 
236 


Fig.  22.     Cabled  Copper  Wire.     Radiograph  taken 
through  the  insulation.     Tube  280 


Fig.  20.     The  substances  most  commonly  used  to  imitate 

diamonds  are  fused  oxide  of  aluminum,  quartz,  and 

lead  glass.  The  figure  shows  a  radiograph  of  these 

three  as  contrasted  with  a  diamond  of  the  same 

size.  In  the  order  of  transparency  to  X-rays 

they  are:    diamond,  fused  oxide  of 

aluminum,   quartz,   lead   glass. 

Tube  21 


Fig.  23.     Cartridge  Fuse.     Tube  280 


Fig.  24.     Fuse  Plug.     Tube  280 


Fig.  21.     Ingersoll  Watch.     Tube  280 


Fig.  25.     "Moulded  Compound"  insulation  around  a 
steel  bolt.     Tube  280 


237 


BY  DR.   WHEELER  P.  DAVEY 

RESEARCH  LABORATORY,  GENERAL  ELECTRIC  COMPANY 

In  an  article  in  the  General  Electric  Review,  January,  1915,  reference  was  made  to 
the  X-ray  examination  of  a  steel  casting  ^  of  an  inch  thick.  Fig.  1  shows  one  of  the 
radiographs  thus  obtained.  All  these  radiographs  showed  plainly  the  tool  marks  on  the 
surface  of  the  casting.  All  but  one  showed  peculiar  markings  which  were  of  such  shape 
as  to  strongly  suggest  that  they  were  indeed  the  pictures  of  holes  in  the  interior.  A 
cylindrical  piece,  one  inch  in  diameter,  was  punched  from  the  casting  at  a  point  where 
the  radiograph,  shown  in  Fig.  1  ,  indicated  that  a  blow-hole  should  be  found.  (The  loca- 
tion of  the  sample  punched  out  is  indicated  by  a  circle.)  Fig.  2  is  a  photograph  of  the 
side  of  the  punching  and  it  shows  the  hole  that  was  found. 

Since  that  article  was  written  it  has  seemed  desirable  (1)  to  obtain  data  from  which 
the  exposure  necessary  for  any  thickness  of  steel  could  be  at  once  calculated,  (2)  to  find 
the  thickness  of  the  smallest  air  inclusion  which  could  be  radiographed  in  a  given  thick- 
ness of  steel,  (3)  to  find  the  direction  from  which  to  hope  for  further  progress,  and  (4)  to 
find  the  technique  of  radiographing  metals. 

In  order  to  gain  some  preliminary  data,  several  pieces  of  ^  in.  boiler  plate  were  ob- 
tained, five  by  seven  inches  in  size.  In  one  of  these,  holes  were  drilled  in  such  a  way 
that  the  axis  of  each  hole  was  midway  between  the  faces  of  the  steel  and  parallel  to 
those  faces.  The  diameters  of  these  holes  are  listed  in  Table  I. 

TABLE  I 


Hole  Number 


Diameter 


J4  inch 
1/8  inch 
TS  inch 
3^  inch 
^i  inch 


Exposures  were  made  on  Seed  X-ray  plates  at  a  distance  of  20  inches  with  Coolidge 
tube  X-117  which  was  operated  on  a  Scheidel-Western  inducation  coil  having  a  mer- 
cury turbine  break.  The  X-ray  plate  was  placed  on  a  sheet  of  J/g  in  lead.  The  steel 
plate  was  laid  on  this  and  a  lead  cover  was  placed  over  the  whole  in  such  a  manner  that 
the  cover  and  backing  made  a  complete  lead  shield  for  the  X-ray  plate.  (See  Fig.  3.) 
A  rectangular  hole  in  the  cover  allowed  such  X-rays  as  were  able  to  penetrate  the  steel 
to  reach  the  X-ray  plate.  This  afforded  complete  protection  against  secondary  rays. 
Without  such  precautions,  the  effect  of  secondary  rays  on  the  X-ray  plate  would  hav^ 
been  greater  than  that  of  the  rays  used  to  take  the  picture.  If  the  steel  had  been  two 
or  three  feet  square,  such  precautions  would  have  been  unnecessary.  By  placing  the 


*  Copyright,  1915,  by  General  Electric  Review. 

239 


pieces  of  boiler  plate  on  top  of  each  other  any  thickness  of  steel  desired  could  be  ob- 
tained. Exposures  were  made  at  11-,  13-  and  15-in.  parallel  spark-gap  between 
points.  „  An  attempt  was  made  to  use  a  17-in.  spark  gap,  but  was  abandoned  due  to 
flashing  in  the  tube.  The  results  are  tabulated  in  Table  II. 


TABLE  II 


Thickness  of 
Steel  in  In. 

Plate 

Spark  Gap 

Exposure  in 
Milliampere-                                     Holes  Visible 
minutes 

H 

D 

11 

. 

7 

1-2-3-4-5 

A 

13 

4 

1-2-3-4-5 

B 

15 

2 

1-2-3-4-5 

i 

E 

11 

45   ' 

1-2-3-4-5 

F 

13 

19 

1-2-3-4-5 

G 

15 

10 

1-2-3-4-5 

IK 

H 

11 

45 

1-2-3  very  faint 

I 

13 

30 

1-2-3  very  faint 

K 

13 

90 

1-2-3  faint 

J 

15 

30 

1-2-3—4-5  very  faint 

L 

15 

60 

1-2-3-4-5  faint 

Fig.  1.    Radiograph  of  a  Steel  Casting  Showing  Flaw  Within  Casting.    The  Circle  Shows  where  a  Piece  was  Later  Punched  Out 

This  really  means,  of  course,  that  at  13-in.  spark-gap  90  milliampere-minutes  is  suf- 
ficient to  enable  one  to  notice  the  difference  in  blackening  between  exposures  through 
1  y^  in.  and  1^  in.  of  steel,  but  is  not  sufficient  to  enable  one  to  detect  the  difference  in 
blackening  between  exposures  through  l^f  in.  and  ll/2  in- 

These  results  were  necessarily  incomplete,  since  the  plates  were  by  no  means  all  of 
the  same  density.  They  served,  however,  to  demonstrate  two.  facts. 

240 


1.  With  the  voltages  which  can  now  be  used,  it  is  impracticable  to  radiograph 
through  more  than  ll/%  in.  of  steel  with  tungsten  target  tubes  because  of  the  time  re- 
quired. 

2.  The  use  of  high  voltages  does  not  seem  to  appreciably  reduce  the  clearness  of  the 
picture  obtained.    (It  was  to  have  been  expected  from  published  data  on  scattering  in 
aluminum  that  enough  scattered  radiation  would  have  been  produced  to  blur  the  pic- 
tures, but  plate  B  apparently  shows  as  good  detail  as 

does  plate  D.) 

It  remained  to  verify  these  conclusions  by  data 
of  a  quantitative  nature.  Seed  X-ray  plates  were, 
therefore,  exposed  under  the  same  conditions  as 
before  except  that  none  of  the  slabs  of  steel  used  had 
been  drilled.  For  each  thickness  of  steel,  all  the 
exposures  at  a  given  spark-gap  were  made  on  the  same 
plate.  Each  plate,  then,  showed  a  series  of  steps 
which  increased  in  density  from  one  end  of  the  plate 
to  the  other.  Thickness  of  steel,  spark-gap,  and 

milliampere-minutes  were  recorded  on  each  plate  by  means  of  lead  numbers, 
regarding  these  plates  is  listed  in  Table  III. 

TABLE  III 


Fig.  2.     Ordinary  Photograph  of  One  Edge  of 

the  Punching  from  the  Plate  Shown 

in  Fig.  2.     Note  Flaw 


Data 


Plate  Xo. 


Thickness  of  Steel 


Spark-gap 


216 

K                                                     H 

inches 

217 

'  H                                                   13 

inches 

218 

Yi    '                                               15 

inches 

221 

1                                                       11 

inches 

220 

1                                                       13 

inches 

219 

1                                                       15 

inches 

222 

IK                                                     15 

inches 

Fig.  3.     Diagram  of  the  Method  of  Preparing  Steel  Sample,  X-ray  Plate,  Lead  Mask  and  Lead  Backing  for  Taking  Radiograph 

A  study  of  these  plates  showed  the  following  facts:  Let  E\/2  be  the  exposure  in 
milliampere-minutes  necessary  to  produce  a  given  darkening  of  the  plate  through  J^ 
inch  of  steel,  and  let  E\  and  EI%  be  the  exposures  necessary  to  produce  the  same  dark- 
ening through  1  inch  and  1%  inch  respectively. 


241 


Then  at  11  in.  gap 
Eg:£i-i:ll 

At  13  in.  gap 

£,,-:£i  =  l:S 
At  15  in.  gap 

'E^:Ei  —  Ei:Eii^  =  1 :8 
Also,  through  both  %  inch  and  1  inch  of  steel 

£.    E*  _  1    •   I 

13  in.  gap  •  -c'   II  in.  gap  —  i  •  "t 

and  E  15in.  gap:-E  13  in.  gap  =  2:3 


Fig.  4a.      Radiograph  of  Autogenous  Weld  in  Steel.     Sample 
No.  1.     Only  the  Surfaces  Have  Been  Welded 


Fig.  5a.     Radiograph  of  Autogenous  Weld  in  Steel. 

Sample  No.  2.     Holes  in  Center  Due  to  Metal 

not  Being  Thoroughly  Fused 


It  is  at  once  evident  that  either  X-rays  from  a  tungsten  target  at  13  in.  gap  are  more 
penetrating  than  when  produced  at  11  in.  gap,  or  the  X-ray  plates  used  are  more  sen- 
sitive to  the  rays  produced  at  13  in.  gap.  There  is  other  evidence  to  show  that  the  first 
of  these  conclusions  is  the  more  probable,  but  it  is  the  effect  of  the  X-rays  on  the  plate 
which  is  of  prime  importance  in  this  work,  so  that  from  a  radiographic  standpoint  we 
may  say  that  in  any  case  the  effective  penetration  of  the  rays  is  a  little  greater,  at  13  in. 
gap  than  at  11  in.  gap. 


X 


Fig.  4b.     Diagram  of  Section  of  Weld  in  Sample  No.  1 


W      1 


Fig.  5b.     Diagram  of  Section  of  Weld  of  Samp'e  No.  2 


In  the  same  way  we  may  conclude  that  the  effective  penetration  at  15  in.  gap  is  the 
same  as  at  13  in.  gap.  There  is,  however,  a  marked  decrease  in  the  amount  of  exposure 
required  as  the  voltage  across  the  tube  (as  measured  by  the  spark-gap)  is  increased. 
This  may  be  due  to  one  of  two  causes,  either  the  efficiency  of  transformation  from  the 
kinetic  energy  of  the  cathode  stream  into  the  energy  of  the  X-rays  may  be  greater  at 
high  voltages,  or  there  may  be  some  peculiarity  in  the  wave-form  produced  by  the  in- 
duction coil  such  that  a  great  deal  of  energy  is  given  off  at  a  voltage  corresponding  to 
13  in.  gap  when  the  coil  is  operated  so  as  to  give  a  maximum  voltage  corresponding  to  a 

242 


15  in.  gap.  Investigation  work  on  crystal-reflection  of  X-rays  will  serve  to  decide  be- 
tween the  two  hypotheses. 

From  the  data  at  hand,  it  is  easily  possible  by  well  known  means  to  construct 
formulae  for  computing  the  exposure  necessary  for  radiographing  steel  at  various 
spark  gaps. 

Let  Qo  be  the  quantity  of  X-rays  impinging  on  the  steel  during  the  exposure. 

Let  Q  be  the  quantity  of  the  rays  which  pass  through  the  steel. 

Let  x  be  the  thickness  of  the  steel. 


Fig.  6a.     Radiograph  of  Autogenous  Weld  in  Steel. 
Sample  No.  3.     Weld  is  Porous 


Fig.  7a.     Radiograph  of  Autogenous  Weld  in  Steel. 
Sample  No.  4.     A  Good  Weld 


Let  X  be  the  coefficient  of  absorption  of  the  steel.    Then,  if  the  X-rays  are  homoge- 
neous, 

Q  =  Q06~X* 

Where  c  is  the  base  of  natural  logarithms. 

E.      Ei 

Now  at  15-in.  gap  we  know  that— r^  =  — —  =  J/g 

E\      Eii 

Ei 

The  rays  given  off  at  15  in.  gap  are,  therefore,  practically  homogeneous.   Since  — -  =  y% 


\ 


Fig.  6b.     Diagram  of  Section  of  Weld  in  Sample  No.  3 


Fig.  7b.     Diagram  of  Section  of  Weld  in  Sample  No.  4 


at  13  in.  gap,  we  may  assume  that  these  rays  are  also  practically  homogeneous.  Rays 
given  off  at  1  1  in.  gap  are  still  sufficiently  homogeneous,  after  having  passed  through 
the  first  few  hundredths  of  an  inch  of  steel,  to  allow  of  being  treated  as  though  they  were 
actually  homogeneous.  Calculations  for  exposures  at  1  1  in.  gap  are  to  be  considered  as 
being  only  good  approximations. 


log  S=i^X  =  2.079 

X  =  4.  1  6  inches^1  =  1  .64  centimeters^1 


243 


Likewise  for  15  in.  gap 

X  =  4.16  inches-1  =  1.64  centimeters-1 
Applying  the  same  method  for  11  in.  gap 

X  =  4.80  inches-1  =  1.89  centimeters-1 

Now  at  15  in.  gap  and  20  in.  distance,  0.8  milliampere-minutes  gives  a  good  ex- 
posure through  Yi  inch  of  steel.  A  corresponding  darkening  would  have  been  produced 
on  a  bare  (unobstructed)  plate  by  an  exposure  of  0.1  milliampere-minutes.  This  cor- 
responds to  Q  in  the  formula.  We  may  therefore  write,  since  <20  =  E, 

0.1  =£e-4-16* 
10£=e4'16* 
log  e  10  £  =  4.16* 
logic   10  £=1.80* 

£=1/10  log"1!,,  1.80* 
where  x  is  the  thickness  of  the  steel  in  inches  or 

E  =  1/10  log"1!,,  0.71  oc 

where  x  is  the  thickness  of  the  steel  in  centimeters. 
The  corresponding  formulae  for  13  in.  gap  are 

£  =  3/20  log"1™  1.80  x  (x  in  inches) 
£  =  3/20  log"1 10  0.71  x  (x  in  cemtimeters) 
The  approximate  formulae  for  1 1  in.  gap  are 

£  =  3/5  log"1™  2.09  x  (x  in  inches) 

£  =  3/5  log'1™  0.82  x  (x  in  centimeters) 

It  remained  to  find  the  thickness  of  the  smallest  air-inclusion  which  could  be  radio- 
graphed in  steel  at  15-in.  gap.  For  this  purpose  two  plates  of  steel  were  taken.  The 
faces  were  machined  flat  and  in  one  of  them  a  slot  was  cut,  thus  giving  a  wedge  of  air. 
The  slot  and  the  faces  of  the  steel  plates  were  then  ground  smooth.  When  completed, 
each  plate  was  ^g  in.  thick.  The  air  wedge  was  10  inches  long,  1  inch  wide,  and  ^  inch 
thick  at  its  thick  end.  When  the  two  plates  were  bolted  together,  the  air  wedge 
simulated  a  blow-hole  in  a  casting.  The  wedge  was  then  radiographed  at  15  in.  gap. 
When  the  X-ray  plates  were  dry  the  place  was  noted  at  which  the  outline  of  the  wedge 
was  barely  visible.  In  order  to  avoid  error,  only  a  small  portion  of  the  wedge  image 
was  viewed  at  one  time,  the  remainder  being  blocked  off  with  cardboard.  It  was 
found  that  an  air  inclusion  0.021  inch  thick  could  be  detected  in  134  inches  of  steel. 
In  ^g  inches,  an  air  inclusion  of  0.007  inch  could  be  detected. 

Besides  the  work  that  has  been  outlined  herein  much  more  has  been  done  in  the 
actual  taking  of  pictures  so  that  the  technique  of  radiography  through  metals  might 
be  worked  out. 

A  record  of  a  single  example  will  suffice.  Four  samples  of  autogenous  welds  in  steel 
were  obtained.  The  welding  had  been  done  with  an  oxy-acetylene  flame.  The  samples 
were  %  inch  thick  and  about  4  inches  square,  and  their  faces  were  fairly  rough.  Sam- 
ple No.  1  had  only  been  welded  on  the  surfaces.  (See  Fig.  4b.)  Sample  No.  2  had  been 
insufficiently  heated  so  that  there  was  incomplete  fusion  of  the  metal  at  the  center. 
(See  Fig.  5b.)  In  welding  sample  No.  3  an  excess  of  oxygen  had  been  used  in  the  flame 
which  caused  the  presence  of  oxide  on  the  surface.  (See  Fig.  6b.)  Sample  No.  4  was 
considered  to  be  a  good  weld.  (See  Fig.  7b.)  One-half  of  each  face  of  the  samples  was 
machined  off,  so  that  half  the  length  of  the  weld  was  between  flat,  parallel  faces;  the 
other  half  was  left  under  the  original  rough  surfaces.  As  a  result,"  one-half  of  each  sample 

244 


was  Yi  inch  thick  and  the  other  half  was  about  %  inch  thick.  Radiographs  were  taken  at 
15-in.  gap  under  the  conditions  described  above.  Reference  to  the  formula  for  exposure 
at  15-in.  gap  shows  that  the  exposures  through  the  ^2  inch  and  ^g  inch  portions  were  in 
the  ratio  of  1  to  1.7.  The  resulting  radiographs  are  shown  in  Fig.  4a,  5a,  6a,  and  7a. 

Fig.  4a  shows  clearly  the  unwelded  center  of  sample  No.  1  in  both  portions  of  -the 
picture.  Fig.  5a  shows,  in  both  portions  of  sample  No.  2,  the  holes  caused  by  the  metal 
not  having  been  thoroughly  fused  at  the  center.  That  portion  of  Fig.  6a  which  was 
taken  through  the  machined  end  of  the  weld  of  sample  No.  3  would  seem  to  indicate  a 
porous  structure.  Such  a  structure  was  evident  during  the  machining.  The  portion  of 
the  picture  taken  through  the  unmachined  end  of  the  weld  did  not  show  such  a  structure 
with  certainty.  This  was  to  have  been  expected,  as  the  inequalities  in  thickness  due  to 
the  uneven  surface  were  at  least  as  great  as  those  due  to  porous  or  frothy  structure. 
Fig.  7a  shows  that,  as  far  as  gross  structure  is  concerned,  sample  No.  4  was  a  good  weld. 

It  is,  of  course,  self-evident  that  a  radiograph  gives  only  the  gross  structure  of  the 
metal,  and  gives  no  information  as  to  the  "grain,"  crystal  interlocking  at  the  edge  of 
the  weld,  etc.  A  radiograph  does,  however,  give  valuable  information  as  to  the  presence 
of  blow-holes,  slag  inclusions,  porous  spots,  and  defects  of  like  nature  which  could  not 
be  found  otherwise  except  by  cutting  into  the  metal.  Unfortunately,  no  fluorouscopic 
screen  now  known  is  sentitive  enough  for  this  work;  therefore,  all  work  in  metals  must 
be  done  radiographically.  An  inspection  of  the  formulae  that  have  just  been  derived 
demonstrates  that,  for  the  present  at  least,  radiography  of  steel  is  a  commercial  possi- 
bility only  up  to  thicknesses  of  y%  inch.  For  greater  thicknesses,  the  time  required  is 
rather  great.  The  big  saving  in  time  which  is  gained  by  the  use  of  a  15-in.  spark-gap  in- 
stead of  a  13-in.  gap  makes  it  seem  probable  that  a  further  increase  in  the  voltage  across 
the  tube  would  allow  one  to  radiograph  still  greater  thicknesses  of  steel. 


Fig.  8.     A  Radiograph  of  a  Steel  Casting  Revealing  a  Flaw  in  the  Interior  of  the  Plate 

245 


X-RAY  EXAMINATION  OF  "BUILT-UP"  MICA* 

BY  C.  N.  MOORE 

The  process  of  manufacturing  "built-up"  mica  for  use  as  an  insulating  material  in 
electrical  machinery  consists  essentially  in  pasting  together,  at  an  elevated  tempera- 
ture under  pressure,  thin  fakes  of  mica  with  a  suitable  binder,  planing  down  the  result- 
ing product  to  the  required  thickness,  and  cutting  it  into  sheets  of  the  required  size.  In 
this  process,  certain  defects  which  would  affect  insulating  qualities  of  the  finished  pro- 
duct have  to  be  guarded  against.  Among  these  are  the  presence  of  foreign  materials 
of  a  metallic  nature,  and  of  areas  not  of  the  required  thickness.  In  practice,  these  de- 
fects are  detected  by  subjecting  the  material  to  very  careful  visual  inspection  and  gaug- 
ing with  a  micrometer.  This,  however,  entails  considerable  labor.  The  successful  ap- 
plication of  X-rays  to  the  detecting  of  defects  in  such  materials  as  steel  and  copper  cast- 


Fig.  1.     Arrangement  of  Coolidge  Tube  and  Viewing  Box  for  Inspection  of  Built-up  Mica 

ings,  already  described  in  earlier  issues  of  this  publication,  suggested  the  possibility  of 
utilizing  X-rays  as  a  means  of  increasing  the  efficiency  of  the  regular  inspection  of  mica. 
With  this  end  in  view,  Dr.  Davey  and  the  writer  obtained  micas  (some  known  to  be 
good  and  others  known  to  be  defective)  for  examination  in  the  Research  Laboratory. 
These  samples  were  about  0.032  of  an  inch  in  thickness  and  had  been  cut  into  small 
sheets  of  the  required  size  for  placing  in  the  commutators.  These  pieces  were  placed 
upon  a  fluorescent  screen  in  a  specially  designed  viewing  box  (Fig.  1)  at  a  distance  of  20 
inches  from  a  Coolidge  X-ray  tube.  When  the  tube  was  operating  with  a  current  of 
about  6  milliamperes  and  a  parallel  spark  gap  of  six  inches,  the  structure  of  the  mica, 
as  shown  on  the  fluorescent  screen,  could  be  viewed  from  the  outside  of  the  box  by 
means  of  a  mirror  set  at  an  angle  of  45  deg.  to  the  screen. 


Published  in  General  Electric  Review,  March,  1915. 


247 


••-'  ;M. 


248 


Some  of  the  samples  examined  contained  small  particles  of  iron  oxide  not  visible  to 
the  eye  on  the  surface  of  the  sheet  of  mica.  As  iron  oxide  is  much  more  opaque  than 
mica  to  X-rays,  this  material  showed  up  as  black  spots  in  the  image  of  the  mica  on  the 
fluorescent  screen.  Other  samples  examined  contained  small  sections  not  as  thick  as 
the  main  portion  of  the  sheet.  These  sections,  being  more  transparent  to  X-rays, 
showed  up  as  light  spots  in  the  image  on  the  screen.  Samples  of  uniform  thickness 
which  contained  no  foreign  material  gave  images  of  uniform  density  upon  the  screen. 
It  was  found  that  the  examination  could  be  made  very  accurately  and  rapidly,  one 
glance  at  the  image  on  the  screen  being  sufficient  to  detect  the  presence  of  any  defects. 

The  nature  of  the  images  on  the  fluorescent  screen  is  shown  in  the  radiographs. 
These  were  taken  on  Seed  X-ray  plates,  with  an  exposure  of  five  minutes  at  a  distance  of 
30  inches  from  a  Coolidge  tube.  The  tube  was  operated  from  an  induction  coil  on  10 
milliamperes  with  a  parallel  spark  gap  of  four  inches.  Fig.  2  shows  the  radiograph  of 
three  sheets  of  fairly  uniform  thickness.  The  various  flakes  of  mica  which  go  together 
to  make  up  the  finished  sheet  are  plainly  visible.  As  these  flakes  are  in  most  cases  only 
a  few  thousandths  of  an  inch  in  thickness,  this  radiograph  shows  what  small  differences 
of  thickness  may  be  detected  by  means  of  the  X-rays  and  the  fluorescent  screen.  Fig.  3 
illustrates  this  more  clearly.  In  this  case  a  sheet  of  mica  0.050  of  an  inch  thick  was 
planed  down  so  that  successive  sections  were  0.045,  0.035  and  0.020  inch  thick.  The 
radiograph  of  this  sheet  shows  that  a  difference  in  thickness  of  0.005  of  an  inch  may 
readily  be  detected. 

The  ease  with  which  foreign  material  may  be  detected  is  shown  by  Fig.  4.  The 
particles  of  iron  oxide  present  in  this  particular  case  were  not  visible  on  the  surface, 
but  they  are  plainly  visible  as  black  spots  in  the  radiograph  taken  of  the  sheets  of  mica. 

Fig.  5  shows  a  radiograph  of  four  sheets  of  mica  0.032  of  an  inch  thick  with  small 
areas  considerably  thinner  than  the  main  portion  of  the  sheet.  These  thinner  areas 
show  up  as  light  spots  in  the  radiograph. 

The  results  obtained  on  an  experimental  scale  in  the  laboratory  have  demonstrated 
the  adaptability  of  the  X-ray  apparatus  as  a  factory  tool  for  the  inspection  not  only  of 
"built-up"  mica  but  of  any  similar  material  of  not  too  great  a  thickness. 


249 


APPLICATION  OF  THE  COOLIDGE  TUBE  TO  METALLURGICAL 

RESEARCH* 

BY  DR.   WHEELER  P.  DAVEY 

Dr.  Weintraub  in  the  February,  1913,  number  of  the  Journal  of  Industrial  and  En- 
gineering Chemistry  describing  boron  and  its  compounds  says : 

"Boron  suboxide,  a  by-product  obtained  in  the  manufacture  of  boron,  can  be  used 
for  obtaining  high  conductivity  cast  copper.  Copper  cast  without  additions  is  full  of 
pores  and  blowholes,  and,  therefore,  mechanically  unfit  and  of  very  low  electric  con- 
ductivity; the  removal  of  the  gases  from  copper  by  the  known  deoxidizers  is  liable  to 
give  an  alloy  containing  a  small  amount  of  deoxidizer,  an  amount  sufficient,  however, 
to  lower  the  conductivity  of  the  copper  very  considerably.  Boron  suboxide,  however, 
has  the  property  of  deoxidizing  copper  without  combining  \vith  it,  as  boron  suboxide 
has  no  affinity  for  copper.  Tons  of  copper  are  cast  now  by  this  process,  improving 
the  quality  of  the  product  and  at  the  same  time  cheapening  it." 


Fig.  1.     Radiograph  of  a  Block  of  "Unboronized"  (Pure)  Copper  side  by  side  with  a  Block  of  "Boronized" 
(Pure)  Copper.     Note  difference  of  internal  structure. 

In  the  refining  of  copper  for  electrical  purposes,  the  electrically  deposited  metal  is 
melted  in  a  reverberatory  furnace.  A  world  of  delicate  chemical  control  is  connected 
with  this  furnace  refining.  When  ready  to  pour,  the  metal  is  cast  into  open  iron  moulds 
which  give  a  copper  pig  or  bar  of  about  75  Ib.  in  weight. 

If  the  metal  wrere  merely  melted  and  then  poured  the  casting  would  be  full  of  blow- 
holes and  would  be  of  low  electrical  conductivity.  The  molten  copper  is  allowed  to 
oxidize  in  the  furnace  and  the  oxidation  is  augmented  by  air  blown  into  the  metal. 
When  the  melt  contains  five  or  six  per  cent  of  oxide,  the  major  part  of  the  other  im- 
purities have  been  burned  away  and  the  work  of  reduction  is  started.  As  ordinarily 
done,  this  consists  in  the  so-called  "poling."  Green  sticks  are  submerged  in  the  molten 


*   Copyright,  191.3,  by  General  Electric  Review. 


251 


copper  and  the  gases  and  carbon  reduce  the  oxide,  and  such  harmful  products  as  sul- 
phur dioxide  are  driven  out  of  the  metal.  The  proper  time  for  pouring  is  not  that  re- 
presenting complete  reduction  of  all  oxide,  as  it  has  been  determined  by  experience 
that  over-poling  also  gives  a  porous  inferior  ingot. 

It  was  once  believed  that  the  copper  absorbed  carbon  which  in  over-poled  copper 
caused  the  rising  in  the  mold  and  the  porous  condition  when  cast.  Hampe  corrected 
this  idea  and  attributed  the  porous  state  of  over-poled  copper  to  the  effect  of  absorbed 
hydrogen  and  carbon  monoxide.  In  any  case  the  fact  remains  that  if  we  merely  melt 
copper  and  cast  it  we  get  a  porous  casting,  and  if  we  thoroughly  remove  dissolved 
oxygen  by  carbon  or  similar  reducing  agents,  we  also  get  a  porous  casting. 


Fig.  2.     Ordinary  Photograph  of  the  Block  of  Fig.  3.     Ordinary  Photograph  of  the  Block  of 

"Unboronized"  Copper  "Boronized"  Copper. 

The  use  of  the  boron  flux  of  Weintraub  has  done  away  entirely  with  the  difficulty  of 
obtaining  sound  castings  of  high  electrical  conductivity.  It  seemed  interesting  to  illus- 
trate the  effect  on  the  porosity  by  an  investigation,  using  X-rays.  For  this  purpose 
some  high  grade  copper  was  melted  in  the  usual  way  and  poured  into  a  sand  mold  to 
give  a  block  10  by  10  by  %  inches.  Another  portion  was  treated  with  one  per  cent  of 
the  boron  flux  at  the  time  of  pouring  and  was  cast  in  a  similar  mold.  These  two  cast- 
ings were  then  placed  side  by  side  on  an  8  by  10-inch  Seed  X-ray  plate,  22  inches  from 
the  focal  spot  of  a  Coolidge  X-ray  tube  and  exposed  for  two  minutes.  The  current 
through  the  tube  was  2.8  milliamperes  and  the  potential  difference  across  the  tube  cor- 
responded to  a  10-inch  parallel  spark  gap  between  points.  The  resulting  radiograph  is 
shown  in  Fig.  1.  The  copper  cast  in  the  ordinary  way  is  seen  to  be  full  of  pores.  The 
cast  with  the  boron  flux  is  so  perfect  that  no  holes  are  visible.  The  two  castings  were 
then  taken  to  the  machine  shop  and  a  portion  of  the  surface  of  each  was  machined  as 
smooth  as  possible.  Ordinary  photographs  were  then  taken,  see  Figs.  2  and  3.  As  was 
to  have  been  expected  from  the  radiograph,  Fig.  1,  the  holes  were  clearly  visible  in  the 
common  copper.  In  the  "boronized"  copper  the  holes  are  either  entirely  absent  or  are 
microscopic. 

The  advantage  of  the  radiograph  in  experimental  work  is  obvious.  Without  the  use 
of  X-rays  it  is  necessary  to  machine  off  layer  after  layer  of  the  sample  in  order  to  expose 

252 


to  view  any  hidden  defects.  Even  when  this  is  done  it  remains  for  the  experimenter  to 
build  up  a  mental  picture  of  the  defects  in  his  casting  on  the  basis  of  what  he  has  seen  on 
each  of  the  exposed  layers.  From  the  radiograph  it  is  possible  to  see  all  of  these  defects 
at  once  without  destroying  the  casting.  If  it  seems  desirable,  it  is  easily  possible  to 
make  stereoscopic  radiographs  whereby  the  defects  may  be  seen  in  their  entirety  and 
their  depths  easily  estimated.  Such  a  stereoscopic  radiograph  of  a  portion  of  the  pure 
copper  casting  is  shown  in  Fig.  4.  This  figure  should  be  viewed  through  an  ordinary 
stereoscope. 

In  view  of  the  results  shown,  the  X-ray  examination  of  metals  as  a  means  of 
metallurgical  research  seems  to  have  certain  attractive  and  desirable  features  not  found 
in  other  methods  and  to  open  a  wide  field  for  further  work. 


Fig.  4.     Stereoscopic  Radiograph  of  a  Portion  of  the  Block  of  Unboronized  Copper   (Actual   Size). 
When  viewed  through  a  hand-stereoscope  this  shows  the  size  and  relative  depth  of  the  pores 


253 


THE   EFFECT  OF  X-RAYS  ON  THE   LENGTH  OF  LIFE  OF  TRIBOLIUM 

CONFUSUM 

BY  WHEELER  P.  DAVEY 

Introduction 

A  great  deal  of  work  is  reported  in  the  literature  on  the  effect  of  X-rays  on  various 
forms  of  animal  life.  A  study  of  this  literature  shows  that,  interesting  though  the  re- 
sults may  be,  it  is  with  few  exceptions  difficult  to  duplicate  the  experiments  because  the 
physical  data  relating  to  the  dosage  have  been  so  incompletely  given.  Excluding  work 
on  human  beings,  the  following  work  may  be  mentioned. 

Hastings,  Beckton  and  Wedd1  found  that  the  hatching  of  silkworm  eggs  was  ac- 
celerated by  X-rays  (dose  not  given)  and  that  the  second  generation  was  less  fertile. 
Bordier2  X-rayed  six  silkworms,  giving  them  a  dose  of  7  to  8  H|,  at  some  unknown  pene- 
tration. He  found  an  increased  restlessness  and  smaller  size.  The  cocoon  was  only  half 
size,  and  the  moth  did  not  emerge.  Hasebroeck3  was  able  to  kill  caterpillars  of  Charaxes 
(dose  not  given),  but  those  of  Vanessa  urticae,  after  being  X-rayed  an  unknown 
amount,  developed  into  butterflies  which  were  unable  to  fly.  Lopriori4  found  a  destruc-. 
tive  action  on  Vallisneria  spiralis,  Genista  and  Darlingtonia  (dose  not  given). 

Perthes5  X-rayed  the  ova  of  Ascaris  megalocephala,  giving  them  a  dose  of  24  H  at  a 
voltage  corresponding  to  6.5  cm.  spark  gap.  Cell-division  was  so  much  retarded  that 
although  the  control  specimens  were  in  the  4-cell  stage  at  the  end  of  30  hours,  the  rayed 
specimens  were  still  in  the  1-cell  and  2-cell  stage.  Hastings6  rayed  eggs  of  the  same 
species,  using  an  unknown  quantity  of  characteristic  X-rays  from  copper.  He  found 
a  retardation  in  growth.  Runner7  X-rayed  the  eggs  of  the  cigarette  beetle  (Lasioderma 
serricorne)  using  a  Coolidge  tube  and  giving  a  dose  of  150  milliampere-minutes  at  65 
kilovolts  at  a  distance  of  7.5  inches.  He  found  that  eggs  less  than  3  days  old  failed  to 
hatch,  and  became  shrunken  in  about  10  days.  Eggs  over  3  days  old  hatched  but  the 
larvae  never  reached  the  pupa  stage.  Larvae  similarly  rayed  refused  to  eat,  and  al- 
though they  lived  a  long  time  after  raying,  they  never  reached  the  pupa  stage. 

Gilman  and  Baetjer8  found  that  X-rays  (amount  not  given)  accelerated  the  develop- 
ment of  eggs  of  Amblystoma  for  about  10  days,  but  produced  monsters.  They  also 
rayed  hen's  eggs  (dose  not  given)  and  found  an  accelerated  development  for  36  hours 
hours  with  the  final  production  of  monsters.  Bordier  and  Galimard9  gave  incubating 
hen's  eggs  daily  doses  of  X-rays  of  15  H  each  (penetration  not  given)  for  20  days.  The 
eggs  contained  no  embryos.  In  specimens  which  had  been  allowed  to  develop  some- 

t  Holzknecht  units  are  measured  by  means  of  the  change  in  color  produced  by  X-rays  in  a  pastille- of  barium  platino- 
cyanide.  The  reading  of  these  pastilles  varies  considerably  with  the  wave-length  of  X-rays  used,  so  that  X-ray  measure- 
ments made  by  such  pastilles  are  meaningless  except  when  the  voltage  across  the  tube  is  given,  or  when  the  "penetration" 
of  the  rays  is  given  in  some  other  reliable  way.  A  better  method  of  measuring  X-rays  is  given  later  in  this  article. 

(0    Hastings,  Beckton  and  Wedd,  Arch.  Middlesex  Hosp.  llth  Cancer  Report,  1912. 

(2)  Bordier,  Le  Radium,  II,  p  410,  1905. 

(3)  Hasebroeck,  Fortschr.  a.d.  Geb.  der  Roent.  XI,  p.  53. 

(<)  Lopriori,  cit.  Schaudin  in  Pflueger's  Arch.  77,  p.  31,  1899. 

t»)  Perthes,  Deut.  med.  Woch.  30.  1904. 

(6)  Hastings,  Arch.  Middlesex  Hosp.  llth  Cancer  Report,  1912. 

(7)  Runner,  Jour,  of  Agr.  Research,  June  12,  1916. 

(*)   Gilman  and  Baetjer,  Am.  Jour.  Physiol,  X,  p.  222,  1904. 
(9)    Bordier  and  Galimard,  Jour,  d'  Elect.  Med.  p.  491,  1905. 

255 


what  before  raying,  growth  was  arrested  at  the  first  dose.  These  results  were  con- 
firmed by  Gaskell10,  but  he  makes  no  record  of  the  dose. 

Lengfellner11  X-rayed  three  pregnant  guinea  pigs,  three  days  before  terra  (dose  not 
given) .  Two  of  the  mothers  were  at  once  killed.  Their  young  died  in  10  minutes.  The 
third  mother  had  a  miscarriage  in  5  hours ;  the  young  were  all  dead.  He  also  rayed  one 
hind  leg  of  an  8  days  old  puppy  (dose  not  given).  Seven  and  one-half  months  later  this 
leg  was  8  cm.  shorter  than  the  other.  Cohn12  rayed  the  heads  of  pregnant  rabbits  (dose 
not  given)  enclosing  the  rest  of  the  rabbit  in  a  lead  box.  Pregnancy  continued  to  full 
term.  For  14  days  after  birth  the  young  seemed  normal,  but  afterward  they  became 
stunted  so  that  after  7  weeks  they  were  only  one-third  the  size  of  the  controls.  Forster- 
ling13  found  that  if  he  rayed  the  heads  of  40-hour  old  rabbits  (dose  not  given)  the  whole 
animal  was  stunted,  but  if  any  other  part  of  the  animal  were  rayed,  only  that  part 
was  stunted.  Krukenberg14  found  that  if  the  pelvis  of  a  young  dog  or  goat  is  X-rayed 
(dose  not  given)  the  growth  of  the  hind  legs  is  retarded.  Raying  the  shoulders  caused 
ataxia  and  nervousness,  affected  the  eyesight  and  made  the  animal  more  irritable. 

The  work  showing  the  possibility  of  a  stimulating  effect  on  eggs  is  confirmed  in  an 
interesting  way  by  studies  on  single  types  of  cells  in  animals.  Menetrier  and  Mallet15, 
and  Rowntree16  have  shown  by  raying  the  ears  and  tails  of  rats  that  somewhere  between 
zero  dose  and  that  dose  necessary  to  produce  dermatitis,  there  is  a  dose  which  stimu- 
lates the  growth  of  epithelial  tissue.  Benjamin,  Ruess,  Sleuka  and  Schwartz17,  Auber- 
tin  and  Beaujard18  and  Murphy  and  Norton19  have  shown  that  X-rays  in  the  proper 
amount  may  increase  the  number  of  leukocytes. 

All  the  above  work  may  be  summarized  as  follows:  X-rays  may  act  upon  an  or- 
ganism (or  on  a  single  type  of  cell  in  that  organism)  in  one  of  three  ways:  (1)  to  pro- 
duce a  stimulation;  (2)  to  produce  a  destructive  effect  which  takes  place  only  after  a 
certain  latent  interval;  (3)  to  produce  an  instant  destructive  effect. 

By  analogy  with  the  action  of  various  drugs,  one  would  expect  that  the  rays  could 
be  made  to  act  in  any  one  of  these  three  ways  at  will  by  merely  varying  the  size  of  the 
dose.  Not  enough  of  the  authors  cited  above  have  adequately  recorded  the  dose  to 
enable  one  to  verify  this  analogy  without  further  experimentation.  It  is  the  purpose  of 
this  article  to  record  the  results  of  experiments  made  toward  this  end. 

About  a  year  ago  the  writer  was  engaged  in  some  preliminary  work  on  the  lethal 
effect  of  X-rays  on  tribolium  confusum.  These  little  bettles  are  ordinarily  called 
"flour  weevils"  and  are  said  by  Chittenden20  to  be  the  most  injurious  enemy  to  pre- 
pared cereal  foods.  In  two  years  from  the  time  of  their  recognition  as  a  distinct  species, 
they  had  spread  to  nearly  every  state  in  the  Union,  and  even  as  early  as  1895  are  said  to 
have  cost  the  millers  of  the  United  States  over  $100,000  in  manufactured  products 
alone.  It  was  found  possible  to  destroy  the  eggs  of  these  beetles  with  X-rays,  thus  giv- 
ing hope  of  a  new  technical  use  for  X-rays,  but  the  most  interesting  results  from  a 
scientific  point  of  view  were  obtained  from  the  beetles  themselves. 

It  was  found  that  these  beetles  cculd  be  killed  with  X-rays  if  a  sufficiently  large  dose 
were  given,  but  it  was  noticed  that  the  beetles  did  not  die  for  several  days  after  they 

(10)  Gaskell,  Proc.  Roy.  Soc.  B  83,  Feb.  28,  1911. 

(»)  Lengfellner,  Munch,  med.  Woch.  p.  44,  1906. 

(12)  Cohn,  Verh.  d.  deutch.  Roent.  Gesel.  Bd  III,  0.128. 

(13)  Forsterling,  Verh.  d.  deutsch.  Roent.  Gesel.  Bd  III,  p.  126. 
(u)  Krukenberg,  Verh.  d.  deutsch.  Roent.  Gesel.  Bd  V.  p.  68. 

(15)   Menetrier  and  Mallet,  Bull,  de  I'Ass.  fran.  pour  letude  du  Cancer  II,  p.  150,  1907. 

('«)   Rowntree,  Arch.  Middlesex  Hosp.  Cancer  Reports  1908-1909. 

(17)  Benjamin,  Reuss,  Sleuka,  and  Schwartz,  Wien  klin.  Woch.  19,  p.  788,  1906. 

(1S)  Aubertin  and  Beaujard,  Arch,  de  Med.  Exper.  et  d'Anat.  Path.  2,  p.  273,  1908. 

(19)  Murphy  and  Norton,  Science,  Dec.  10,  1915. 

(20)  Chittenden,  Bull.  No.  4,  New  Series,  Revised  Ed.  U.S.  Dept.  of  Agr.  p.  113,  1902. 

250 


were  rayed.  Further  experiments  indicated  that  the  length  of  this  latent  interval 
depended  upon  the  amount  of  the  X-ray  dose,  and  there  seemed  to  be  some  evidence 
that  this  relation  was  approximately  logarithmic.  It  was,  therefore,  decided  to  repeat 
these  experiments  more  carefully,  first  making  sure  that  the  effect  was  really  due  to 
X-rays  and  not  some  attendant  circumstance,  and  then  to  investigate  the  relation  be- 
tween the  latent-interval  and  the  X-ray  dosage.  This  required  a  large  number  of 
beetles  and  it  was  necessary  to  determine  their  life  history  and  how  best  to  propagate 
them.  It  was  found  that  they  grow  and  propagate  best  in  oatmeal  or  whole  wheat  flour, 
but  they  will  live  in  corn  meal,  white  flour  or  any  of  the  prepared  cereal  products. 
Propagation  takes  place  best  at  a  temperature  of  35-36  deg.  C.  and  at  high  humidity. 
Temperatures  of  45  deg.  C.  or  over  are  fatal.  The  eggs  are  white  and  from  0.3  to  0.6 
mm.  in  diameter.  They  are  usually  associated  with  pieces  of  grain.  Larvae  grow  to  a 
length  of  5  or  6  mm.  and  shed  their  skins  six  times.  Pupae  are  white.  Young  beetles 
are  a  light  straw  color  which  later  darkens  to  a  russet.  The  beetles  are  about  4  mm. 
longer.  They  are  especially  adapted  to  such  work  as  is  reported  here  because  they 
are  small,  harmless,  easy  to  handle  and  count  in  large  numbers;  they  propagate 
readily,  cannot  crawl  out  of  glass  beakers  or  small  porcelain  crucibles,  and  show  little 
tendency  to  fly. 

In  the  preliminary  experiments  mentioned  above,  the  beetles  were  packed  in  small 
wooden  pill  boxes  with  some  food.  There  were  25  beetles  in  each  box.  There  was  a 
possibility  that  death  was  not  due  to  any  action  of  X-rays  on  the  beetles,  but  might 
have  occurred  from  any  of  the  following  causes : 

1 .  Lack  of  air  and  food. 

2.  High  temperature  due  to  over-crowding. 

3.  Injury  due  to  over-crowding. 

4.  XO-2  caused  by  the  high  voltage  connections  of  the  X-ray  tube. 

5.  Ionized  air. 

6.  Excessive  humidity. 

7.  Effect  of  X-rays  on  the  food  in  the  boxes  or  even  on  the  boxes  alone. 

The  following  seven  experiments  were  therefore  made : 

Experiment  I 

To  Show  that  the  Beetles  were  not  Killed  by  Lack  of  Air  and  Food  Rather  than  by  X-rays 

A.  10  beetles  were  sealed  up  in  a  glass  tube  with  a  rubber  stopper,  without  food,  in  a  space 

0.05  cubic  inch.     They  were  all  alive  at  the  end  of  76  hours. 

B.  20  beetles  were  sealed  up,  without  food,  in  a  space  0.05  cubic  inch.     They  were  all  alive 

at  the  end  of  3  weeks,  but  some  of  them  seemed  to  be  stuck  together  by  a  film  of 
moisture.     At  the  end  of  4  weeks,  8  were  still  alive. 

C.  10  beetles  were  sealed  up  in  a  space  0.05  cubic  inch  together  with  a  pinch  of  white  flour. 

They  were  all  alive  at  the  end  of  8  weeks.     Evidently  the  flour  had  taken  care  of  the 
body  moisture  noted  in  B. 

Therefore,  Tribolium  confusum  are  not  easily  killed  by  lack  of  air,  provided  they  are  kept 
dry  and  have  food. 

Experiment  II 

To  Show  that  the  Beetles  were  not  Killed  by  some  Effect  of  Temperature,  Rather  than  by  X-rays 

20  beetles  were  placed  in  a  round-bottom  test  tube,  %  of  an  inch  in  diameter,  well  heat- 
insulated.  Temperature  was  measured  from  time  to  time  by  means  of  a  delicate 
thermocouple.  The  highest  temperature  reached  was  27 1/±  deg.  C. 

Therefore,  the  beetles  cannot  by  over-crowding  in  a  heat-insulated  space  raise  their  tem- 
perature high  enough  to  produce  death. 

257 


Experiment  III 

To  Show  that  the  Beetles  tyere  not  Killed  by  Mechanical  Injury  Rather  than  by  X-rays 

A.  5  beetles  were  placed  in  a  tapered  test  tube.     Diameter  of  test  tube  was  %  inch.     Length 

of  taper  was  1  ^  inch.  A  small  hole  in  the  bottom  gave  ventilation.  The  beetles  were, 
therefore,  crowded  together,  and  repeatedly  crawled  over  one  another  in  their  efforts 
to  escape.  All  were  alive  at  the  end  of  97  hours. 

B.  5  beetles  were  shaken  violently  in  a  glass  beaker  50  times  a  day.    At  the  end  of  two  days, 

4  were  still  alive.     At  the  end  of  a  week,  3  were  still  alive. 

C.  In  addition,  it  may  be  stated  that  beetles  used  as  "controls "  in  the  main  body  of  this  work 

do  not  seem  to  be  at  all  affected  by  being  dropped  through  a  funnel  several  times 
daily  during  the  process  of  counting. 
Therefore,  the  beetles  are  not  easily  given  fatal  injuries. 

Experiment  IV 

To  Show  that  the  Beetles  were  not  Killed  by  NO?  Rather  than  by  X-rays 

A.  20  beetles, were  put  in  a  test  tube  %  inch  in  diameter  with  a  little  corn  meal.    The  test  tube 

was  connected  to  a  source  of  NO*..  All  beetles  were  alive  after  having  been  in  an 
atmosphere  of  NO*,  for  25  hours.  At  the  end  of  64  hours,  5  were  alive. 

B.  25  beetles  were  put  in  a  vial  and  exposed  to  dilute  NO*,  for  5  minutes.     The  concentration 

of  NO*  was  such  as  to  distinctly  color  starch-KI  paper  in  1  Yi  minutes.  23  beetles 
were  found  alive  after  the  20th  day.  But  there  is  not  enough  ozone  and  NO*  to- 
gether in  the  lead  box  where  the  beetles  are  X-rayed  to  color  starch-KI  paper  during 

an  exposure  of  14,000  at  50  kv. 

«o 

Therefore,  there  is  not  enough  NO*,  produced  while  the  beetles  are  being  X-rayed  to  affect 
them. 

Experiment   V 

To  Show  that  the  Beetles  were  not  Killed  by  Ionized  Air  Rather  than  by  X-rays 

25  beetles  were  carefully  shielded  from  X-rays.  Ionized  air,  together  with  what  little 
ozone  and  NO*,  might  be  present  was  drawn  past  the  beetles  during  an  exposure  of 

MAM 
19,600  at  50  kv.     Even  if  only  10  per  cent  of  the  ions  remained  uncombined 

when  they  reached  the  beetles,  still  this  would  be  equal  to  that  caused  directly  by 

MAM 
1960  •  at  50  kv.    This  dose  of  X-rays,  acting  directly  on  the  beetles,  would  have 

<0O 

killed  them  all  in  less  than  2  weeks  if  death  were  produced  by  ionized  air  rather  than 
by  X-rays  directly.     But  at  the  end  of  21  days,  only  1  beetle  was  dead. 
Therefore,  the  beetles  are  not  killed  by  ionized  air. 

Experiment   VI 

To  Show  that  the  Beetles  were  not  Killed  by  too  High  Humidity  Rather  than  by  X-rays 

A.  10  beetles  were  put,  without  food,  in  a  flat  bottomed  test  tube  %  inch  in  diameter,  which 

was  kept  dry  by  a  side  tube  filled  with  P*O$.    All  but  one  were  alive  after  6  days. 

B.  60  beetles  were  gathered  in  such  a  way  that  each  beetle  was  slightly  moistened  on  the 

back.  They  were  all  put,  without  food,  in  a  test  tube  1  inch  in  diameter.  As  they 
crawled  over  each  other,  the  moisture  was  spread  over  their  whole  bodies.  In  6  hours 
most  of  the  beetles  were  dead.  Those  alive  were  so  weak  that  they  could  not  turn 
over,  even  when  lying  on  their  sides.  60  other  beetles  put  in  a  similar  test  tube  with 
a  cloth  bottom,  lived.  The  cloth  bottom  could  only  have  acted  as  a  ventilator  and  an 
absorber  of  water. 

C.  Beetles  are  grown  in  an  almost  water-saturated  atmosphere  in  the  brooders  and  seem  to 

thrive  well. 

Therefore,  the  beetles  are  not  harmed  by  either  extreme  dryness  or  by  high  humidity,  but 
may  be  killed  by  strangulation  when  water  is  condensed  on  them. 

Experiment   VII 

To  Find  Effect  of  X-rays  on  the  Food  of  the  Beetles 

A  box  similar  to  those  used  in  the  preliminary  experiments  was  filled  with  corn  meal  and 
X-rayed  15,000  milliampere-minutes  at  25  cm.  distance  at  50  kilovolts.  25  beetles 
were  then  put  in  this  box  with  the  cornmeal.  They  lived  lives  of  normal  length.  But 
beetles  rayed  this  amount  die  almost  instantly. 

Therefore,  X-raying  the  boxes  and  the  food  has  no  effect  upon  the  length  of  life  of  the 
beetles. 

»     258 


In  the  light  of  the  above  experiments,  it  seems  safe  to  conclude  that  the  death 
of  the  beetles  recorded  below  was  due  to  X-rays,  rather  than  to  some  accidental 
circumstance. 


/?LftO . 

Transformer 


-*-To  Vo/tmete. 


APPARATUS 

X-rays  were  produced  by  a  water-cooled  Coolidge  tube  (tungsten  target)  operating 
directly  from  a  high-tension  60-cycle  transformer.  Such  tubes  will  rectify  their  own 
current  up  to  50-100  milliamperes,  at  50  kilovolts  (r.m.s.).  Oscillograph  tests  showed 
that  the  transformer  was  of  such  a  type  that  when  operated  under  the  above  conditions 
the  wave-form  of  the  secondary  (high  voltage)  resembled  that  of  the  primary  (low 
voltage) ,  and  the  inverse  voltage  did  not 
exceed  the  direct  voltage  (i.e.  operating 
voltage  of  the  tube)  by  5  per  cent. 
Further  tests  with  the  oscillograph 
showed  that  the  r.m.s.  voltage  of  the 
secondary  differed  from  that  shown  by 
a  voltmeter  coil  by  not  more  than  3 
per  cent.  Tube  voltage  was,  therefore, 
measured  in  terms  of  r.m.s.  kilovolts,  as 
shown  by  the  meter. 

The  voltage  impressed  upon  the 
primary  was  controlled  by  means  of  an 
auto-transformer  of  such  size  as  to  cause 
no  appreciable  change  in  wave-form. 
The  wave-form  used  was  very  nearly 
sinusoidal. 

The  filament  of  the  X-ray  tube  was 
heated  by  current  from  a  small  trans- 
former. This  was  connected  through  a 
ballast  transformer  to  the  terminals  of 
the  circuit  supplying  the  auto-trans- 
former. Connections  are  Shown  in  Fjg  x  Diagram  of  connections  of  X-ray  Outfit 

Fig.  1. 

Tube  current  was  measured  with  a  direct-current  milliammeter.  Since  this  current 
was  pulsating  (half  of  every  wave  being  suppressed  by  the  rectifying  action  of  the  X-ray 
tube) ,  oscillograph  records  were  taken  to  compare  the  meter-reading  with  the  instanta- 
neous value  of  the  current.  It  was  found  in  all  cases  that  the  value  of  the  mean  current 
as  shown  on  the  meter  was  almost  exactly  half  the  value  of  the  peak  of  the  wave. 
The  current  through  the  tube  was,  therefore,  read  in  mean  milliamperes  on  the  meter. 
The  wave  form  of  this  current  was  similar  in  every  way  to  that  of  other  Coolidge 
tubes. 

The  X-ray  tube  was  in  a  lead  box  whose  walls  were  %  inch  thick.  This  provided  a 
safe  protection  from  X-rays  at  the  voltages  used.  The  tube  was  connected  to  the 
transformer  by  ^2  inch  rods,  to  prevent  corona.  Where  these  rods  entered  the  lead 
box,  the  lead  was  replaced  by  lead  glass  1  inch  thick,  which  acted  both  as  insulation  and 
as  X-ray  protection. 

A  chamber  of  lead  (see  A,  Fig.  1)  7  cm.  square  and  5  cm.  long  was  placed  in  the  wall 
of  this  lead  box,  directly  opposite  the  focal  spot  of  the  X-ray  tube.  A  sheet  of  aluminum 


259 


0.025  mm.  thick  was  fastened  across  the  end  of  the  chamber  nearest  the  X-ray  tube. 
This  protected  the  interior  of  the  chamber  from  electrostatic  effects,  and  prevented  any 
NOz,  ozone,  etc.,  from  the  interior  of  the  lead  box,  and  any  radiant  heat  from  the  X-ray 
tube  from  entering  the  chamber.  The  lead  sides  of  the  chamber  protected  the  interior 
from  any  secondary  X-rays  which  might  be  produced  on  the  walls  of  the  lead  box.  The 
only  rays  which  could  enter  the  chamber  were  those  sent  out  directly  from  the  X-ray 
tube  itself. 

Into  this  chamber  were  placed,  four  at  a  time,  the  boxes  of  beetles  to  be  rayed. 
These  boxes  were  of  wood,  cylindrical  in  shape,  1  Y%  inches  in  diameter  and  Y%  of  an  inch 
high.  The  wood  was  %  of  an  inch  thick.  Each  box  contained  25  beetles  and  a  little 
cornmeal,  and  was  kept  closed  during  the  raying.  The  X-rays,  therefore,  after  leaving 

the  X-ray  tube,  passed  through  0.025  mm.  of  Al 
and  3  mm.  of  wood  before  reaching  the  beetles  and 
cornmeal.  At  the  voltage  employed  in  this  work 
(50  kv.r.m.s.),  the  error  due  to  absorption  of 
X-rays  by  the  small  thickness  of  Al  and  wood  was 
very  small. 

EXPERIMENTAL 

Two  or  three  thousand  beetles  were  gathered 
from  the  same  brooder  on  the  same  day  and  put 
into  a  large  granite-ware  pail.  The  next  morning 
they  were  packed  with  a  little  sterile  cornmeal  in 
the  wooden  boxes  mentioned  above,  25  in  each  box. 
In  this  way  the  distribution  of  age,  susceptibility 
to  X-rays,  etc.,  was  as  nearly  uniform  as  possible. 
From  this  time  on  the  tightly  closed  boxes  were 

kept  in  incubators  at  35-36  deg.  C.  and  at  saturated  humidity,  except  while  being 
X-rayed  or  while  being  counted.  Every  box  was  opened  daily,  the  beetles  separated 
from  the  cornmeal  and  a  record  made  of  the  number  of  live  and  dead  beetles. 
The  assistants  who  did  this  counting  has  no  way  of  knowing  the  dose  of  X-rays 
which  had  been  given. 

After  all  the  beetles  in  a  given  group  of  boxes  were  dead,  the  data  sheets  were 
collected  and  the  data  combined  as  shown  in  Table  I.  From  4  to  8  control  boxes 
were  used  with  each  experiment  to  make  sure  that  the  beetles  were  in  every  way 
normal.  The  normal  death  rate  at  the  end  of  the  first  15  days  was  never  more 
than  4  per  cent. 

MAM 
Beetles  rayed  500  at  50  kv.  were  practically  all  dead  in  15  days.      Beetles 

JjO 

rayed  larger  doses  were  all  dead  in  less  than  15  days.  Therefore,  no  correction  for 
normal  death  rate  of  the  X-rayed  beetles  was  considered  necessary. 

It  was  found  that,  while  all  the  beetles  in  a  given  box  did  not  die  at  the  same 
moment,  there  was  a  very  narrow  range  of  time  during  which  most  of  them  died,  thus  sug- 
gesting that  we  were  dealing  with  a  quantitative  effect  which  could  be  studied  to  some 
good.  For  example,  the  results  given  more  in  detail  in  Table  I  and  II  and  in  Fig.  2-A 

MAM 

show  that  if  the  dose  was  15,000  -        -  at  50  kv.,  all  the  beetles  were  dead  at  the 

2o2 

MAM 

end  of  the  raying;  if  the  dose  was  2000  at  50  kv.  practically  all  the  beetles  died 

zo 


Fig.  2-A.     Curves  Showing  Per  Cent  Beetles 
Dead  at  Different  Intervals  after  Raying. 


Doses  Vary  from  500  to  8000 


at  50  Kv. 


260 


between  the  3rd  and  the  6th  days  after  raying,  while  half  of  them  died  between  the  4th 

MAM" 

and  5th  days;  if  the  dose  was  500     __„  -  at  50  kv.,  death  took  place  between  the  4th 

2o2 

and  the  9th  days,  while  half  of  them  died  between  the  6th  and  8th  days.     Doses  less 

MAM 

than  500  -       -  at  50  kv.  were  not  fatal  to  all  the  beetles. 
2o2 

Now  if  the  percentage  of  dead  beetles  is  plotted  against  the  time  which  has  elapsed 
since  they  were  X-rayed,  it  is  evident  that  the  points  follow  a  smooth  curve,  which  is 
the  integral  of  a  probability  curve.  If  now  the  slope  of  this  curve  is  plotted  against  its 
absicissae,  a  probability  curve  may  be  obtained.  If  the  beetles  represented  by  two 
such  curves  have  been  gathered  from  the  same  brooder  at  the  same  time,  the  cor- 
responding points  on  the  two  curves  may  be  compared,  for  they  represent  beetles  of 
corresponding  resistance  to  the  action  of  the  X-rays.  It  will  be  noticed  that  the  curve 
approaches  the  zero  and  the  100  per  cent  lines  asymptotically.  This  is  in  agreement 
with  the  well-known  fact  in  toxicology  that  some  individuals  are  especially  susceptible 
to  a  given  harmful  agent,  so  that  a  very  small  dose  causes  death,  while  other  individuals 
are  especially  resistant  to  the  same  agent  so  that  they  continue  to  live  for  a  com- 
paratively long  time,  even  when  given  large  doses.  The  steepness  of  the  curves  as 
plotted  in  Fig.  2-A  is  a  measure  of  the  idiosyncrasy. 


SCO 


/,000 


LOG  DOSE 


Fig.  2-B.     Curves  Showing  Length  of  Life,  after  Raying,  of  10,  25,  50,  75  and  90  Per  Cent 
of  Beetles.     "Days  Life"  is  Plotted  against  the  X-Ray  Dose. 


This  gives  a  method  of  handling  the  data  which  not  only  eliminates  errors  due  to 
idiosyncrasy,  but  which  even  gives  a  measure  of  idiosyncrasy  which  is  accurate  enough 
between  the  limits  of  "25  per  cent  dead"  and  "75  per  cent  dead."  Fig.  2-A  shows  a 
series  of  results  obtained  in  this  way  in  which  even  the  10  per  cent  and  90  per  cent 
points  could  be  used.  Before  discussing  Fig.  2-B,  it  will  be  necessary  to  explain  the 
methods  of  recording  X-ray  dosage. 

In  order  to  define  the  quantity  of  X-rays  and  the  bundle  of  wave-lengths  used  in  a 
given  experiment,  it  is  necessary  to  record  explicitly, 

1 .  The  material  used  as  a  target  in  the  X-ray  tube. 

2.  The  thickness  and  kind  of  filters  (if  any). 

3.  The  form  of  the  voltage  wave. 

4.  The  form  of  the  current  wave. 


261 


TABLE  I 


MAM 
Beetles  Rayed  2000     25,      %  50  Kvrms 


Box                  Days 

0 

i 

2 

3.2 

4.2 

4.8 

5.2 

6 

7 

7.2 

36  

0 

0 

1 

1 

5 

14 

17 

22 

24 

25 

37  

0 

o 

0 

o 

6 

15 

19 

24 

25 

25 

38  

0 

0 

0 

0 

5 

16 

18 

24 

25 

25 

39  

0 

0 

1 

1 

9 

15 

18 

24 

25 

25 

Total  dead  

0 

0 

2 

2 

25 

60 

72 

94 

99 

100 

Per  cent  dead  

0 

0 

2 

2 

25 

60 

72 

94 

99 

100 

TABLE  II 


MA  M 
Beetles  Rayed  500     25,      @  50  Kvrr 


Box                 Days 

0 

l 

2 

3 

4 

5 

6 

7 

8   8.3 

9 

10 

11 

23  

0 

0 

1 

1 

1 

2 

11 

18 

24   24 

24 

25 

^Fi 

24  

0 

0 

o 

o 

o 

1 

5 

9 

17   20 

23 

24 

25 

25  

0 

0 

0 

0 

1 

3 

6 

19 

22  :  24 

25 

25 

?5 

26  

0 

o 

1 

1 

1 

1 

6 

11 

20   23 

25 

25 

?5 

Total  dead  

0 

o 

2 

2 

3 

7 

28 

57 

83   91 

97 

99 

100 

Per  cent  dead  

0 

0 

2 

2 

3 

7 

?8 

57 

83   91 

97 

9P 

100 

The  following  must  be  recorded  either  explicitly  or  implicitly: 

5.  Voltage  across  the  X-ray  tube. 

6.  Current  through  the  tube. 

7.  The  length  of  time  the  X-rays  were^mployed. 

8.  The  distance  from  the  focal  spot  of  the  X-ray  tube  to  the  point  to  be  rayed. 
If  1,  2,  3,  4  are  kept  constant  throughout  the  experiment,  they  may  be  stated  once 

for  all  (as  was  done  in  this  report  under  the  head  of  "apparatus"),  and  the  dose  of 
X-rays  may  then  be  denned  by  either  of  two  methods : 

a.  The  voltage  may  be  expressed  directly,  or  an  approximation  may  be  given  in  terms  of  the 

readings  of  a  Benoist  penetrometer  or  in  terms  of  the  Christen  "half  value  layer." 
The  other  factors  may  be  given  in  terms  of  the  reading  of  a  Kienbock  strip  or  a  Holz- 
knecht  pastille,  etc.,  or  better. 

b.  The  voltage  and  distance  are  given  directly  and  the  product  of  the  current  and  time  is 

given,  thus, 
"  100  milliampere-minutes  at  25  cm.  distance  at  50  kilovolts."    This  is  usually  contracted 


to  read 


MAM 
100     252     at  50  kv. 


It  will  be  noticed  that  the  distance  is  expressed  in  terms  of  its  square, 
cause  the  intensity  of  X-rays  varies  inversely  as  the  square  of  the  distance 
stress  cannot  be  laid  upon  the  necessity  for  recording  the  voltage,  and  for 
voltage  reading  constant ;  for  not  only  does  the  penetrating  power  of  the  X- 
upon  the  voltage,  but  even  the  quantity  of  rays  given  off  by  the  tube  per 
depends  very  largely  upon  the  voltage. 

In  Fig.  2-B,  days  life  is  plotted  against  the  logarithm  of  the  X-ray  dose. 
50,  75  and  90  per  cent  points  of  the  curves  for  500,  1000,  2000  and  4000 


This  is  be- 
Too  much 
keeping  the 
rays  depend 
milliampere 

The  10,  25, 
kv.  lie  on  a 


262 


family  of  straight  lines  V  =  A — B  log  X,  where  X  is  the  X-ray  dose  and  Y  is  the  number 
of  days  life  after  raying.    All  these  lines  meet  the  zero  line  at  the  point  64,000.    The 

MAM 


points  for  8,000  and  15,500 


at  50  kv.  do  not  lie  on  these  lines,  but  when  taken 


252 

MAM 
along  with  the  points  for  4000  —          at  50  kv.,  they  are  found  to  form  a  new  family  of 

*^f_7 

curves  of  the  same  type  as  the  first,  but  with  a  steeper  slope.    The  interpretation  of  this 
is  given  later.     It  should  be  noted  here,  however,  that  beetles  rayed  more  than  4000 

MAM 

— ~ —  at  50  kv.  were  unable  to  move  their  legs  and  antennae  easilv,  but  that  this  effect 
25s 

was  not  noticed  in  beetles  rayed  less  than  this  amount.    For  this  reason  it  was  difficult 

at  the  higher  dosages  to  obtain  data  as  accurate  as  that  obtained  at  the  lower  dosages . 

Curves  like  Fig.  2  have  been  obtained  time  after  time,  the  only  difference  being  in 


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3:  -"Curves  Similar  to  those  of  Fig.  2-B.     "Days  Life"  is  Plotted 
against  the  Logarithm  of  the  X-Ray  Dose 


the  height  of  the  ordinates  and  the  slope  of  the  family  of  curves.  Fig.  3  shows  a  typical 
curve  of  this  sort.  It  will  be  noticed  that  in  spite  of  the  difference  in  the  ordinates,  the 
sharp  break  in  the  curve  occurs  at  the  same  dosage. 

At  50  kilovolts  the  lowest  dose  of  X-rays  which  is  fatal  to  all  the  beetles  is  500 

MAM 

-_,     •    In  order  to  explore  the  field  below  this  dose,  1 100  beetles  were  gathered  from 

the  same  brooder  at  the  same  time  and  packed  into  boxes  of  25  each  with  sterile  corn- 
meal,  and  these  boxes  were  divided  into  1  groups  of  8  boxes  each,  and  one  group  of  4 
boxes. 

One  group  of  8  was  kept  as  a  control.    The  others  were  rayed  100,  200,  250,  300 
MAM  MAM 


~rr~  at  50  kv.  respectively.    The  group  of  4  was  rayed  500 


at  50  kv.     The 


results  are  plotted  in  Fig.  4,  curves  A,  B,  C,  D,  E,  F,  G. 

It  will  be  noticed  that  there  is  very  little  difference  between  curves  A  and  B.  Ex- 
cept for  a  small  hump  between  0  and  10  days,  there  is  no  essential  difference  between 
curves  A  and  C.  This  is  brought  out  in  curve  G,  which  is  the  same  as  curve  C,  except 
that  the  calculations  are  based  on  the  supposition  that  there  were  no  beetles  dead  on  the 
tenth  day.  This  similarity  is  still  more  marked  between  curves  B  and  G.  Except  for 
a  similar  hump  between  0  and  12  days,  curve  D  resembles  curves  A  and  B.  The  hump 

263 


is,  however,  much  higher  than  in  C.  In  curve  E  the  hump  is  still  of  the  same  shape  as  in 
curves  C  and  D  (it  is  the  integral  of  a  probability  curve)  and  covers  a  period  of  12  days, 
but  is  considerably  higher.  In  curve  F  the  "hump"  is  the  whole  curve,  except  for  a 
very  flat  portion  which  represents  a  single  very  resistance  beetle. 

These  curves  have  been  duplicated  several  times,  and  although  beetles  gathered  from 
different  brooders  at  different  times  give  curves  of  slightly  different  shape,  still  all  the 
curves  agree  very  closely  with  the  typical  ones  shown  in  Fig.  4,  especially  with  regard 
to  the  "hump." 

This  would  make  it  seem  that  at  50 

MAM 

kilovolts,  200 


252 
lethal  dose  of   X-rays 


is  the  minimum 
for 


the  least 

MAM 

resistant  beetles,  and  that  500          2 

£o 

is  the  minimum  lethal  dose  for  the 
most  resistant  beetles.  The  fact  that 
the  "hump"  is  always  of  the  same 
form  suggests  that  these  beetles  which 
would  live  9  days  with  a  dose  of  200 

MAM 

at  50  kv.  would  live  a  shorter 


MAM 


252 

time  if  rayed  250  -       -  at  50  kv.  and 
2o2 

that  some  more  slightly  resistant 
beetles  which  would  be  unaffected  by 
a  dose  of  200  are  killed  off  at  the  end 
of  9  days  by  a  dose  of  250.  But 
MAM 


when  a  dose  of  500 


252 


at  50  kv 


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Pays 

Fig.  4.     Curves  showing  per  cent  beetles  dead  at  different 

intervals  after  raying.     Doses  vary  from 

MAM 


0  to  500 


at  50  kv. 


is  reached,  the  most  resistant  beetles 
are  also  affected  by  the  rays,  so  that 
the  whole  graph  then  approximates 
the  probability  integral. 

Due  to  a  breakdown  of  the  trans- 
former, the  data  to  date  at  any  other 
voltage  than  50  kv.  is  fragmentary. 
Fig.  5  shows  the  data  obtained  at  68 
^'^RMS  Just  before  the  transformer 
broke  down.  The  beetles  were  gath- 
ered at  the  same  time  as  those  of  Fig.  3 

and  the  raying  was  done  within  3  days  of  that  of  Fig.  3.  The  two  graphs  may,  therefore, 
be  compared  for  what  they  are  worth.  It  is  hoped  later  to  determine  more  accurately 
the  effect  of  voltage. 

THEORETICAL 

It  has  been  shown  above  that  if  the  dosage  of  X-rays  is  sufficiently  large,  the  experi- 
mental relation  between  length  of  life  (Y)  and  X-ray  dosage  (X)  is  of  the  form 

Y  =  A-B  logX 

This  formula  may  be  easily  derived  from  an  extension  of  the  Psycho-physic  Law, 
which  states  that  a  change  in  response  to  an  external  stimulus  is  directly  proportional  to 


26  i 


the  change  in  the  stimulus,  but  inversely  proportional  to  the  amount  of  the  stimulus. 
Thus  the  flicker-sensation  caused  by  suddenly  dimming  a  light  is  directly  proportional 
to  the  amount  of  dimming,  but  inversely  proportional  to  the  total  intensity  of  the  light. 
Now  let  us  suppose  that  the  same  principle  applies  to  the  action  of  X-rays  on  living  cells. 

Let  F  =  the  number  of  days  a  beetle  will  live  after  being  X-rayed. 

Let  X  =  the  amount  of  the  X-ray  dose. 

Then  dY  is  directly  proportional  to  dX  and  inversely  proportional  to  X.  Moreover, 
an  increase  in  X  produces  a  decrease  in  Y. 

Therefore, 

dX 


dY=-B- 


X 


Integrating,  Y  =  A—B  (log  X)  which  is  the  same  as  the  equation  of  the  experimental 
graph. 

The  constant  of  integration  A  has  at  pre- 
sent only  a  theoretical  meaning,  for  it  repre- 
sents the  number  of  days  a  beetle  would  live  if 

MAM 
it  were  X-rayed  only  1      ._„     at  50  kv.  and 


Days 
IA\ 


13 


12 


/O 


252 

if  no  process  of  repair  went  on  inside  the 
beetle,  and  if  there  were  no  other  cause  of 
death  present. 

It  will  be  noticed  that  this  formula  takes 
no  account  of  any  cause  of  death  other  than 
X-rays,  nor  of  any  process  of  repair  which 
may  go  on  inside  the  beetle.  It,  therefore,  ap- 
plies only  when  the  X-ray  dose  has  been  large 
enough  to  completely  destroy  the  protective 
mechanism  of  the  beetle,  and  when  the 
damage  caused  by  the  X-rays  is  large  enough 
so  that  all  other  causes  of  death  may  be 
neglected. 

It  should  be  noted  that  length  of  life  after 
raying  is  a  measure  of  the  resistance  of  an 
organism  to  X-rays,  not  of  its  susceptibility. 
If  X-rays  are  ableto  kill  the  organism  in  two 
different  ways,  as  by  attacking  two  different 

kinds  of  cells,  or  two  different  organs,  then  the  experimental  graph  should  be  the 
resultant  of  two  straight  lines,  but  in  such  a  case  it  must  be  remembered  that 
susceptibilities  not  resistances  are  to  be  added. 

The  graphs  shown  above  seem  to  indicate  that  Tribolium  Confusum  are  affected 

MAM 
in  two  ways  by  the  X-rays;  the  threshold  dose  being,  for  the  first  way,  500          —  at 

MAM 

50  kv.  and  for  the  second  way  about  4000         2     at  50  kv.    These  graphs  would  also 

seem  to  indicate  that  the  cause  of  death  for  dosages  between  500  and  4000  is  negligible 

MAM 
in  the  presence  of  that  for  dosages  above  4000  at  50  kv. 


Fig    5.     Curves  giving    fragmentary  data  of  "  Days 

Life"  plotted  against  the  logarithm  of  the 

X-rays  dose  at  68  kv. 


265 


SUMMARY 

1.  It  has  been  shown  that  the  lethal  effect  noticed  on  tribolium  confusum  beetles 
after  X-raying  is  really  due  to  X-rays  and  not  to  some  accidental  circumstance. 

2.  A  method  has  been  developed  which  eliminates  the  error  due  to  idiosyncrasy, 
thus  making  it  possible  to  obtain  bio-physical  data  of  a  considerable  degree  of  precision. 

3.  It  has  been  shown  that  the  lethal  effect  of  X-rays  on  tribolium  confusum  bears  a 
definite  mathematical  relation  to  the  logarithm  of  the  total  X-ray  dose. 

4.  An  extension  of  the  Psycho-physic  Law  gives  a  theoretical  explanation  of  the 
experimental  data,  if  the  resistance  rather  than  the  susceptibility  of  the  organism  to  the 
X-rays  is  considered. 

The  effects  produced  by  small  doses,  by  changes  in  voltage,  and  by  dividing  the  dose 
of  X-rays  into  small  parts,  will  be  taken  up  in  a  later  article. 

A  full  bibliography  of  work  done  up  to  1912  may  be  found  in  Fortschr.  a.d.  Gebiete 
der  Roent.  XIX,  p.  123,  1912,  in  an  article  by  Walter. 

A  complete  bibliography  of  all  X-ray  work  since  1912  has  been  published  by  Gocht. 


266 


PROLONGATION   OF   LIFE   OF   TRIBOLIUM   CONFUSUM   APPARENTLY 
DUE  TO  SMALL  DOSES  OF  X-RAYS 

BY  WHEELER  P.  DAVEY 

In  a  previous  article*  by  the  author  experiments  were  described  which  showed: 
first,  that  X-rays  when  given  in  sufficient  quantity  were  able  to  shorten  the  life  of 
tribolium  confusum;  and  second,  that  the  length  of  life  after  X-raying  could  be  ex- 
pressed by  a  mathematical  formula,  the  theoretical  derivation  of  which  was  given.  It 
•is  the  purpose  of  this  article  to  give  the  results  of  further  experiments  showing  that  it  is 
apparently  possible  to  materially  lengthen  the  life  of  these  same  organisms  by  giving 
sufficiently  small  doses  of  X-rays. 

In  the  article  referred  to,  curves  were  given  showing  that  the  minimum  dose  neces- 

MAM 

sarv  to  kill  all  the  beetles  was  500  -       -  at  50  kv.f   Some  of  the  less  resistant  beetles 

252 

MAM' 

could  be  killed  bv  smaller  doses,  but  the  curves  for  100  and  200  -       -  at  50  kv.  had 

252 

portions  in  which  the  death  rate  was  lower  than  that  of  the  controls.  Comment  on  this 
was  reserved  until  it  could  be  confirmed  by  further  experiments.  Ample  confirmation 
has  now  been  obtained. 

The  experiments  undertaken  fall  into  two  groups :  A ,  those  in  which  very  small  doses 
of  X-rays  were  given  daily  throughout  the  life  of  the  beetles;  B,  those  in  which  the 
X-ray  dose  was  given  all  at  one  time,  as  in  the  work  previously  published.  In  each  of 
these  groups  of  experiments  it  has  been  shown  possible  to  duplicate  results  time  after 
time,  subject  only  to  those  general  limitations  which  are  inseparable  from  biological 
work.  Typical  experiments  in  each  group  will  be  described  in  the  following.  The  ap- 
paratus and  technique  were  the  same  as  in  the  work  previously  reported. 
EXPERIMENT  A:  PROLONGATION  OF  LIFE  DUE  TO  SMALL  DAILY  DOSES  OF  X-RAYS 

Six  groups  of  approximately  950  individuals  each  were  taken.  These  were  known  as 
groups  IV,  IW,  IX,  IY,  IZ,  and  JA. 

Group  /  V  was  the  control. 

MAM 
Group  IW  was  given    6J^     ^y     at  50  kv.,  25  ma.  daily. 

MAM 
IX  was  given  12 y<i     ^~2     at  50  kv.,  25  ma.  daily. 

MAM 
IY  was  given  25          2-2      at  50  kv.,  25  ma.  daily. 

MAM 
IZ    was  given  50         2-2 —  at  50  kv.,  25  ma.  daily. 

MAM 
JA  was  given  100    — 2-2      at  50  kv.,  25  ma.  daily. 

After  159  days  the  beetles  were  practically  all  dead.  The  data  on  the  death  rates 
were  then  collected  and  plotted  as  shown  in  Fig.  1.  These  graphs  furnish  ample  proof 
that  it  is  possible  to  reduce  the  death  rate  of  tribolium  confusum  by  small  daily  doses 
of  X-rays. 


"Effect  of  X-rays  on  the  Length  of  Life  of  Tribolium  Confusum,"  Journal  of  Experimental  Zoology,   Vol.  22, 

Xo.  3,  April,  1917.     See  also  p.  25.5,  this  book. 

t   i-e.,  500  milliampere-minutes  at  25  cm.  distance  at  50  "rootmean-square"  kilovolts. 
Copyright,  1919,  by  General  Electric  Review. 


267 


'/.DEAD 


Table  I  gives  readings  from  these  graphs  to  the  nearest  whole  number.  These 
readings,  taken  from  the  smooth  curves  of  the  graphs,  do  not  differ  from  the  actual  ex- 
perimental data  by  more  than  one  per  cent. 

Except  while  being  X-rayed  or  counted,  the  beetles  were  kept  in  an  incubator  at  34- 
35  deg.  C.  In  order  to  make  sure  that  the  results  were  not  affected  by  some  possible 
"temperature  co-efficient  of  life,"!  the  controls  were  taken  out  of  the  incubator  while 
group  JA  was  being  rayed,  and  were  kept  out  during  the  whole  raying.  Since  group  JA 
was  rayed  the  longest  time  each  day,  this  meant  that  the  controls  were  cooled  off  for  a 
longer  time  than  groups  IW,  IX,  IY,  IZ.  Therefore,  if  cooling  off  for  a  few  minutes 
each  day  happened  to  tend  to  increase  the  length  of  life,  then  the  controls  were  made  to 
live  longer  than  they  otherwise  would.  The  actual  increase  in  length  of  life  observed  in 
groups  IW,  IX  and  IY  is,  therefore,  not  due  to  any  possible  effect  of  temperature,  but 
occurs  in  spite  of  it.  After  so  many  boxes  of  beetles  in  JA  were  dead  that  the  time  of  • 
raying  group  IZ  was  greater  than  the  time  of  raying  JA,  the  controls  were  kept  out 
of  the  incubator  while  group  IZ  was  being  rayed. 

Some  data  not  given  in  the  graphs 
may  be  of  additional  interest.  Each 
group  was  divided  into  two  sub-groups 
of  about  the  same  number  of  individuals 
each.  It  was  found  that  the  idiosyncrasy 
was  great  enough  that  the  curves  of  the 
corresponding  sub-groups  could  not  be 
exactly  superimposed.  However,  it  was 
found  that  this  idiosyncrasy  was  always 
less  than  the  changes  in  death  rate  caused 
by  X-rays.  By  way  of  illustration, 
Table  II  shows  the  percentage  of  beetles 
dead  in  each  sub-group :  on  the  day  when 
50  per  cent  of  the  controls  were  dead,  and 
on  the  day  when  50  per  cent  of  the  X- 
rayed  group  were  dead.  This  table  shows 
that  the  lowest  death  rate  among  the  con- 
trols (group  IV)  was  higher  than  the 
highest  death  rate  among  the  beetles  of 
groups  IW,  IX,  IY. 

It  is  interesting  to  note  in  this  con- 
nection that  the  total  dose  received  by 
these  beetles  was  greatly  in  excess  of  that  minimum  dose  which,  when  given  all  at 
once,  would  have  caused  premature  death. 

TABLE  I 


60    SO     106 

Fig.l 


Number 
Days 

Per  Cent 
Dead 

Per  Cent  Dead 
Group  IW 

Per  Cent  Dead 
Group  IX 

Per  Cent  Dead 
Group  /  Y 
2.MAM 

Per  Cent  Dead 
Group  IZ 
MAM 

Per  Cent  Dead 
Group  JA 

Raying 

Control 

25' 
at  50  kv.  Daily 

"    25* 
at  50  kv.  Daily 

at  50  kv.  Daily 

°    25* 
at  50  kv.  Daily 

2S2 
at  50  kv.  Daily 

10 

17 

17 

14 

11 

12 

20 

20 

34 

29 

25 

21 

28 

69 

30 

46 

35                       30 

28 

39 

79 

40 

51 

42                       36 

34 

oo 

90 

50 

54 

47                       40 

39 

67 

96 

60 

58 

53                       44 

44 

77                        99 

70 

63 

59 

48 

52 

88 

100 

80 

67 

65 

56 

63 

96 

90 

74 

74 

69 

79 

98 

100 

84 

83 

84 

91 

99 

t   Loeb  &  Xorthrup,  Proc.  Nat.  Acad.  Sci.,  Aug.  1916. 


268 


A  further  analysis  of  the  data  of  groups  IV  to  JA  will  be  of  interest.  The  curves 
shown  in  Fig.  1  when  re-plotted  on  probability  paper*  appear  as  shown  in  Fig.  2.  It 
was  found  that  each  curve  was  composed  of  portions  of  three  accurate  probability 
curves  joined  end  to  end.  It  is  as  though  there  were  three  causes  of  death,  or  perhaps 
three  definite  groups  of  ages.  These  three  portions  of  the  death-rate  curve  will  be 
termed  A ,  B,  and  C.  Portion  C  represents  those  beetles  which  lived  the  longest  in  their 
group. 

Table  III  gives  the  death  rate  per  100  in  each  group  for  A,  B,  and  C. 

TABLE  II 


Group 

APPROX.  50  PER  CENT 
CONTROLS  DEAD 

APPROX.  50  PER  CENT 
X-RAYED  BEETLES 
DEAD 

Sub-group  (1) 

Sub-group  (2) 

Sub-group  (1) 

Sub-group  (2) 

39th 

day 

56th  day 

IV                            47.7 
IW                           41.6 

54.2 
42.1 

52.8                                 60.0 
48.1                                 52.3 

39th 

day 

74th  day 

IV 
IX 

47.7 
32.4 

54.2 
38.3 

59.9 
44.7 

70.3 
54.1 

39th 

day 

67th  day 

IV 
IY 

47.7 
31.7 

54.2 
36.7 

58.6 
48.5 

68.2 
50.2 

39th 

day 

38th  day 

IV 
IZ 

47.7 
52.9 

54.2 
54.5 

46.8 
50.1 

53.4 
52.4 

39th 

day 

14th  day 

IV 
JA 

47.7 
88.5 

54.2 
91.7 

22.1 
43.7 

24.9 
46.8 

TABLE  III 


Daily 
Group                                 ^   _[ 

Per  Cent 
Which  Died 

Per  Cent 
Which  Died 

Per  Cent 
Which  Died 

of  "A" 

of  "B" 

of  "C" 

IV 

Control 

44 

26 

30 

IW 

6J4 

32 

36 

32 

IX 

12H 

26 

26 

48 

IY 

25 

21 

35 

44 

IZ 

50 

23 

61 

16 

JA 

100 

64 

17 

19 

*  The  prdinates  of  probability  paper  are  so  spaced  that  the  ordinary  curve  of  the  probability  integral  is  represented  by 
a  straight  line. 


269 


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Fig.  2 


It  is  evident  that  the  smallest  daily  dose 
(group  /IF)  decreases  the  death  rate  of  "A" 
and  that  those  beetles  which  are  kept  from 
dying  of  "/I,"  die  of  "5."  Deaths  from 
cause  "  C  "  are  practically  unaltered.  A  larger 
daily  dose  (group  IX)  causes  about  half  of 
those  which  would  normally  die  of  "A"  to 
die  of  "C."  A  still  larger  daily  dose  (group 
IV)  causes  half  of  those  which  would  have 
died  of  "A"  to  die  of  "5"  and  "C."  A  still 
larger  daily  dose  (group  IZ)  acts  much  like 
the  previous  dose  in  causing  almost  half  of 
those  which  would  have  died  of  "A"  to  die  of 
"I?,"  but  it  differs  from  it  in  that  some  of 
those  which  would  have  died  of  "C"  are 
prematurely  killed.  The  largest  daily  dose 
employed  (group  J  A)  caused  about  a  third  of 
those  which  would  have  died  of  "B  "  and  "C' 
to  die  of  "A." 

It  is  hard  to  interpret  all  this.  It  may  be 
that  life  cannot  exist  except  in  the  presence 
of  a  small  amount  of  radioactivity.  The  radio- 
activity of  the  earth  may  not  have  been  of  the 
optimum  value,  so  that  some  benefit  was  de- 
rived from  the  X-rays  received  each  day. 
The  following  is  an  effort  at  an  alternative  ex- 
planation. The  evidence  given  by  group  JA 
shows  that  the  lethal  action  of  X-rays  is  tied 
up  in  some  way  with  cause  of  death  "A."  It 
is  well  known  that  the  lethal  action  of  X-rays 
is  more  marked  on  cells  in  the  process  of 
division  than  on  those  in  the  resting  state. 
Therefore,  small  daily  doses  (larger  than  a 
certain  minimal  value)  can  kill  off  those  few 
cells  which  happen  to  be  in  a  state  of  division 
at  the  time  of  raying.  The  death  of  these  few 
cells  stimulates  the  production  of  more  to 
take  their  places  between  the  periods  of  ray- 
ing. Therefore,  small  daily  doses,  instead  of 
increasing  the  death  rate  from  cause  ".4," 
actually  decrease  it  by  stimulating  the  pro- 
cesses of  repair.  The  whole  individual  beetle, 
therefore,  has  a  smaller  chance  of  dying  from 
"A"  and  is  compelled  to  die  of  either  "5"  or 
"C."  When  the  daily  dose  is  increased  to 
such  a  value  that  the  daily  destruction  of  cells 
is  equal  to  or  greater  than  the  production  of 
new  cells,  premature  death  occurs,  from  causes 
"£"  or  "A"  (see  groups  IZ  and  JA). 


270 


EXPERIMENT  B:     PROLONGATION  OF  LIFE  DUE  TO  SMALL  SINGLE  DOSES  OF 

X-RAYS 

Five  groups  of  approximately  850  individuals  each  were  taken.    These  were  known 
as  groups  JB,  JC,  JD,  JE  and  JF. 

Group  JB  was  the  control. 

MAM 
Group  JC  was  given  100     9-2     at  50  kv.,  50  ma. 

MAM 

JD  was  given  200  — opT  at  50  kv.,  50  ma. 

MAM 
JE  was  given  300     9-2      at  50  kv.,  50  ma. 


JF  was  given  400 


MAM 


at  50  kv.,  50  ma. 


The  beetles  were  rather  old,  so  that  the  controls  were  all  dead  on  the  40th  day  of  the 
experiment.  There  were  so  few  beetles  still  alive  after  the  35th  day  that  the  results  of 
the  last  five  davs  are  not  of  the  same 


order  of  accuracy  as  those  of  the  first 
35  days. 

During  the  first  10  days  of  the  experi- 

MAM 
ment,  group  JC  (100  at  50  kv.) 

had  the  same  death  rate  as  the  controls. 
After  the  tenth  day  the  death  rate  was 
considerably  less  than  that  of  the  con- 
trols. The  two  groups  were  divided 
into  two  equal  sub-groups  and  although 
it  was  found  that  the  idiosyncrasy  was 
such  that  the  sub-groups  were  not  ex- 
actly alike,  still,  after  the  tenth  day,  the 
highest  death  rate  of  group  JC  was 
lower  than  the  lowest  death  rate  of  the 
controls. 

During  the  first  17  days  of  the  experi- 
ment, group  ]D  (200  -  at  50  kv.) 
252 


/OO 
80 
60 
40 
20 

/OO 
80 
60 

20 

/OO 

80 

40 
20 

100 
80 
60 
40 
ZO 

/OO 
80 
60 
40 
10 


V.D&ID 


~7 


874 


/OO 


0      /0    20    30 


Fig.  3 


had  a  higher  death  rate  than  the  con- 
trols. After  the  17th  day,  the  death 
rate  of  groups  JD  was  less  than  that  of  the  controls.  After  the  20th  day,  the  death 
rate  of  JD  was  identical  with  that  of  JC.  When  divided  into  two  equal  sub-groups 
as  described  above,  it  was  found  that  after  the  22nd  day  the  highest  death  rate  of  group 
JD  was  less  than  the  lowest  death  rate  of  the  controls. 

MAM 
During  the  first  29  days  of  the  experiment,  the  death  rate  of  group  JE  (300 

— ^0 

at  50  kv.)  was  greater  than  that  of  the  controls.    After  the  29th  day,  the  death  rate  of 
JE  was  less  than  that  of  the  controls. 

MAM 

The  death  rate  of  group  JF  (400  at  50  kv.)  was  at  all  times  greater  than 


252 


that  of  the  controls. 


271 


These  results  are  shown  graphically  in  Fig.  3.    Fig.  4  contains  an  analysis  of  these 

same  curves  by  means  of  probability  paper,  showing  that,  as  in  the  case  of  experiment 

A ,  the  curves  are  composed  of  accurate  portions  of  probability  curves  placed  end  to  end. 

All  of  the  foregoing  results  seem  to  be  a  direct  confirmation  of  the  curves  given  in 

the  previous  paper  (loc.  cit.).    The  effect  of  concentrated  single  doses  is  not  nearly  so 

marked  as  the  effect  of  a  series  of  small 
"homeopathic "  doses.  This  seems  to  be 
much  the  same  law  as  is  already  well 
known  in  serum  therapy  and  in  the 
action  of  certain  drugs.  In  the  case  of 
serum  therapy,  this  law  has  been  shown 
to  be  identical  with  the  law  of  absorption. 
If  it  could  be  rigorously  shown  that  the 
effects  of  exposure  to  X-rays  follow  the 
same  general  law,  we  should  conclude 


that  the  X-rays  are  responsible  for  the 
production  of  some  substance,  perhaps 
in  the  blood,  which  is  later  absorbed. 

Summary 

(1)  It  has  been  shown  that  the  life  of 
tribolium  confusum  may  be  prolonged 
by  the  use  of  a  purely  physical  agent; 
i.e.,  X-rays. 

(2)  The  prolongation  of  life  due  to  a 
series   of   small   daily   doses   is   greater 
than  that  of  larger  doses  given  all  at 
once. 

(3)  The  lethal    effect  of  an   X-ray 
dose  is  less  if  it  is  split  up  into  a  series  of 
small  daily  doses,  than  if  it  is  given  all 
at  once. 

(4)  A  method  of  graphical  analysis 
of  results  has  been  described  by  which 
the  number  of  causes  of  death  may  be 
determined  from  the  death  rate,  and  by 
which  the   effect  of  an  external  agent 
upon    each    of    these    causes   may   be 
studied. 


Fig.  4 


(5)  Using  the  same  kind  of  organism  throughout  the  whole  experiment,  the  work  re- 
ported in  this  and  the  previous  paper  (loc.  cit.)  has  shown  that,  by  merely  varying  the 
size  of  the  dose,  a  purely  physical  agent  (X-rays)  may  be  made  to  produce  at  will  (a)  a 
stimulation,  (6)  a  destructive  effect  which  occurs  only  after  a  latent  interval,  and  (c)  an 
instant  destructive  effect. 


272 


X-RAYS 

BY  DR.  WHEELER  P.  DAVEY 

PART  I 

Of  all  the  theories  so  far  proposed  as  to  the  nature  of  X-rays,  the  Electromagnetic 
Theory  seems  to  be  the  most  useful.1  This  theory  seeks  to  account  for  X-rays  in  terms 
of  the  properties  of  the  electrostatic  and  magnetic  fields  of  electrically  charged  moving 
particles.  It  is  found  that  such  a  theory  is  capable  of  giving  a  rational  basis  for  cor- 
.relating  the  facts  known  at  present  about  X-rays  and  is  also  able  to  suggest  fruitful 
topics  for  further  research. 

It  is  well  known  that  if  a  stationary  body,  A,  is  given  an  electric  charge,  the  region 
around  A  becomes  altered  in  such  a  manner  that  a  force  of  repulsion  is  exerted  upon  any 
other  body  similarly  charged.  For  simplicity  let  A  be  spherical  in  shape  and  not 
situated  near  any  other  charged  body;  then,  the  electric  force  extends  from  it  radially 
in  all  directions.  If  the  location  of  A  or  the  amount  of  charge  upon  it  is  changed,  there 
is  a  corresponding  change  in  the  electric  force,  and  this  change  in  the  electric  force  is 
propagated  from  A  with  the  velocity  of  light  (3X 1010  cm.  per  sec.) 

Now  suppose  that  A  is  moving  with  uniform  velocity  and  that  this  motion  has  al- 
ready been  kept  up  for  So  long  a  time  that  the  whole  electric  field  of  force  is  in  motion 
with  it.  This  condition  of  the  electric  field  is  represented  diagrammatically  in  Fig.r  1. 
If  A  is  suddenly  stopped,  the  condition  of  the  electric  field  is  as  represented  in  Fig.  2a. 
The  direction  of  motion  is  represented  by  XX1.  If  A  had  continued  in  uniform  motion  for 
a  time,  t,  it  would  have  reached  A7,  and  would  have  had  a  field  such  as  is  shown  by  the 
lines  A'DE,  A' HI,  A'X',  A'  JK,  etc.  But,  since  A  has  been  stopped,  it  will  finally 
have  a  field  such  as  is  shown  by  the  lines  ABC,  AFG,  AX',  ALM,  etc.  The.  inter- 
mediate stage  is  shown  by  the  full  lines  ABDE,  AFHI,  AX',  ALJK,  etc.,  where \BD, 
FH,  LJ,  etc.,  constitute  the  changing  portion  of  the  electric  field,  which,  as  has  been 
stated  before,  moves  out  from  A  in  all  directions  with  the  velocity  of  light.  BD  has  the 
same  relation  to  ABDE  as  the  "crack  of  a  whip  "  has  to  the  whip  itself. 

The  movement  of  a  line  of  electric  force  calls  into  existence  a  magnetic  field,  and 
this  holds  true  independently  of  whether  the  movement  is  caused  as  previously  de- 
scribed or  is  caused  by  electric  charges  passing  through  a  wire.  Now,  when  a  current 
passes  through  a  wire,  the  direction  of  the  magnetic  field  is  mutually  perpendicular  to 
the  direction  of  the  electric  field  and  to  the  direction  of  the  current.  Since  the  electric 
field  of  each  elementary  charge  of  electricity  moves  with  that  charge,  we  may  make  this 
more  general  statement — The  direction  of  the  magnetic  field  due  to  a  moving  electric  field 
is  mutually  perpendicular  to  the  direction  of  that  electric  field  and  to  the  direction  in  which 
that  field  is  moving.  We  should  therefore  expect  to  find  that  as  BD,  FH,  LJ,  etc.,  move 
outward  from  A  they  would  be  accompanied  by  a  magnetic  field  perpendicular  to  the 
paper.  The  pulse  sent  out  by  A  upon  stopping  is  therefore  partly  electric  and  partly 
magnetic  in  its  nature  and  is  called  an  "electromagnetic"  pulse. 

1  The  word  "useful"  is  used  here  purposely.     No  theory  is  to  be  regarded  as  a  statement  of  absolute  fact.    A  theory 
may  be  disproved,  but  it  can  never  be  completely  proved.     Of  two  or  more  theories,  all  of  which  are  equally  consistent 
with  the  known  experimental  data,  the  one  to  be  preferred  is  that  which  proves  to  be  of  greatest  use  in  producing  results. 
Copyright,  1915,  by  General  Electric  Review. 

273 


According  to  the  electromagnetic  theory,  primary  X-rays  consist  of  such  pulses  as 
have  just  been  described.  If  negatively  charged  particles  of  matter  (variously  called 
corpuscles,  /3  particles,  cathode  rays,  and  electrons)  are  shot  out  at  high  velocities 
toward  the  target  of  an  X-ray  tube,  they  will  experience  a  great  decrease  in  velocity 
upon  entering  the  face  of  the  target;  and  during  the  time  that  this  retardation  occurs, 

X-rays  are  produced.  Their  properties  seem 
to  depend  only  upon  the  rate  of  retarda- 
tion which  the  cathode  rays  experience  at 
the  target,  and  this  in  turn  depends  only 
upon  the  voltage  across  the  terminals  of 
the  X-ray  tube  and  upon  the  material  used 
as  a  target. 

If  OH  (see  Fig.  2)  is  drawn  parallel  to 
XX,  so  as  to  represent  the  path  taken  by  H, 
then  FO  is  called  the  "thickness"  of  the 
pulse  FH.  We  will  call  this  thickness  d.  The 
electric  force  along  FH  may  be  considered 
as  being  the  resultant  of  two  forces,  Pi  along 
FO,  and  P2  along  OH.  Since  the  dielectric 


Fig.  1 


constant  of  empty  space  is  unity,  we  have  at  once. 


Pi  = 


(AF? 


or 


where  e  is  the  amount  of  the  charge  on  A,  and 
_ OH        e       OH 

E>    __  D    \x 

Now  let 

w  =  the  velocity  of  A  just  before 
being  suddenly  stopped. 

V^ihe  velocity  of  propagation  of 
the  pulse  FH  (equal  to  the  velocity 
of  light). 

T  =  the  time  A   would  have   re- 
quired to  have  reached  A '. 
Then 

OH  =  AA'  =  uT 
and  since 

it  follows  that 


r=VT 


OH  = 


ur 
~V 


and 


P2  = 


eu 


'dV    rdV 


Fig.  2 


Since  the  pulse  FH  is  moving  in  a  direction  parallel  to  AF,  we  are  onlv  interested  in 
that  component  of  P2  which  is  parallel  to  AF,  viz.,  P2',  where 

sin  6 


Now  the  energy  of  the  pulse  is  partly  electrostatic,  and  partly  magnetic.    That  portion 

which  is  electrocstatic  is  equal  to— — times  the  square  of  the  electric  force. 

STT 


e2  u-  sin2  0 

r  E-  =  • 
" 


The  magnetic  portion  is  —      :,  times  the  square  of  the  magnetic  intensity.     But  the 

SirV 

magnetic  intensity  is  V  times  the  electric  force  which  produces  it. 

_  e2  w2  si  n?  6 
M~S/rf*.ffiV* 

The  total  energy  of  the  pulse  FH  is  therefore 

„     _  e2  M2  si  n2  e 
hFO~4irr*d*V* 

The  energy  of  the  whole  spherical  pulse  is  obviously  obtained  by  integrating  E/?O'  thus 
giving 

w     2/e2"2 
E=AdV 

where  d  depends  for  its  value  upon  the  values  of  0  and  u  and  upon  the  atomic  weight  of 
the  material  used  as  a  target. 

It  will  be  useful  to  gain  an  idea  of  how  u  depends  upon  the  voltage  across  an  X-ray 
tube.  If  the  charge  on  a  cathode  partical  is  c  then  a  voltage  E  will  represent  an  energy 
of  Ee.  The  mechanical  energy  cf  the  electron  is  %mu2.  By  the  law  of  conservation 
of  energy 

E  e  = 


E-     i/w    2 
E  =    <  —  u2 


For  purposes  of  convemsn^li  this  is  often  written 


Now  the  value  of  */m  is  not  a  constant  but  depends  in  a  complicated  way  upon  the 
value  of  u.  Table  I  has  been  calculated  from  data  given  by  Bucherer  (1909),  Steinmetz 
(1898),  and  Algermissen  (1906).  Spark  gap  lengths  are  to  be  considered  only  as  ap- 
proximate measurements  of  voltage  under  ordinary  working  conditions. 

When  the  cathode  rays  strike  the  target  they  are  not  stopped  instantaneously  at  the 
surface,  but  merely  suffer  retardation  so  that  they  penetrate  for  some  distance  into  the 
body  of  the  target.1  After  once  entering  the  target,  the  particles  no  longer  all  move  in 
the  same  general  direction,  but  travel  more  or  less  radially.  If,  for  a  given  velocity  of 
cathode  rays,  we  imagine  the  target  to  be  made  thicker  and  thicker,  a  thickness  will  be 
reached  at  last  for  which  there  are  as  many  particles  emerging  in  one  direction  as  in 
any  other.  This  thickness  is  called  "The  depth  of  complete  scattering."2 

In  aluminum  it  is  0.015  cm.;  in  copper,  0.001  cm.;  in  silver,  0.001  cm.;  in  gold, 
0.00020  cm.  ;  and  in  lead,  0.00025  cm.  at  90,000  volts.  It  varies  directly  as  the  voltage 
employed  across  the  tube. 

Those  primarv  rays  which  are  able  to  overcome  the  absorbing  effect  of  the  target 
reach  the  'surtetc*.  jnd  emerge  into  the  vacuum  space  of  the  tube.  Measurements  have 


'  W.  R.  Ham,  Phys.  Rev.  xxx,  Jan.,  1910. 

W.  P.  Davey,  Jour.  Franklin  Inst.,  Mar.,  1911. 

L.  G.  Davey,  Phys.  Rev.,  Sept.,  1914. 
-  J.  A.  Crowther,  Roy.  Soc.  Proc.,  80,  A,  pp.  186-206,  Mar.  5,  1908. 

W.  R.  Ham.  Phys.  Rev.  xxx,  1,  pp.  119-121,  Jan.,  1910. 


275 


TABLE  I 


M  in  Miles  per  Sec. 

u  in  Cm.  per  Sec. 

E  in  Volts 

£  in  Cm. 
Spark  Gap  Between 
Xeedle  Points 
(A-C.) 

£  in  Cm. 
Spark  Gap  Between 
Balls  5  Cm.  in 
Diameter  (D-C.) 

11,800 
18,900 
37,900 

X  109 
1.88 
3.00 
6.00 
7.50 

1,000 

2,330 
10.300 
16,200 

0.1 
0.2 
0.8 
1.3 

0.2 
0.4 

56,800 
63,000 

9.00 
9.60 
10.2 
10.8 
11.4 

23,770 
27,200 
30,900 
34,800 
39,300 

2.0 
2.3 
2.7 
3.1 
3.6 

0.7 

0.8 
0.9 
1.0 
1.2 

74,400 

12.0 
12.6 
13.2 
13.8 
14.4 

44,000 
49,100 
54,300 
59,900 
66,100 

4.3 
5.0 
5.9 
6.9 
7.9 

1.4 
1.6 
1.8 
2.1 
2.4 

93,000 

15.0 
15.6 
16.2 
16.8 
17.4 

72,800 
79,700 
87,200 
95,400 
104,200 

9.3 
10.6 
12.0 
13.6 
•    15.5 

2.7 
3.1 
3.6 
4.2 
4.9 

111,600 

18.0 
18.6 
19.2 
19.8 
20.4 

113,400 
123,500 
134,100 
146,100 
158,900 

19.6 
20.0 

5.6 

6.8 
8.1 
10.1 
13.0 

130,200 

21.0 
21.6 
22.2 
22.8 
23.4 

172,900 
188,100 
205,300 
224,000 
245,200 

30.00 
45.0 

16.2 
19.5 

142,600 

24.0 
24.6 
25.2 

268,900 
296,000 
327,700 

shown1  that  if  the  voltage  across  the  tube  is  made  very  small,  then  the  primary  rays 
at  the  moment  of  generation,  have  their  maximum  of  intensity  in  a  directiori  perpendi- 
cular to  the  cathode  stream,  and  a  minimum  of  intensity  in  a  direction  parallel  to  the 
cathode  stream.  This  effect  is  called  "polarization."  As  the  voltage  across  the  tube  is 
increased,  the  polarization  is  decreased,  until  finally  it  becomes  immeasurable.  This  is 
explained  by  assuming  that  at  the  higher  potentials  the  rays  formed  by  the  initial  re- 
tardation of  the  cathode  stream  are  negligible  in  their  effects,  when  compared  with 
those  rays  which  come  out  in  all  directions  from  the  depth  of  complete  scattering. 

SECONDARY  RAYS 

When  X-rays  are  made  to  impinge  upon  a  substance,  that  substance  itself  becomes 
a  source  of  X-rays,  which  are  called  "secondary  rays."  Two  distinct  types  of  secondary 
rays  are  recognized,  viz.,  "scattered"  and  "characteristic".  When  X-rays  pass  through 
a  substance,  the  emergent  beam  is  found  to  act  in  the  same  way  that  light  acts  on  pass- 
ing through  a  fog.  The  rays  retain  all  the  peculiarities  of  the  incident  beam,  but  have 
suffered  a  diffuse  reflection  or  "scattering."  Scattered  rays  emerge  from  both  the  in- 

1  R.  Blondlot,  Comptes  Rendus,  186,  pp.  284-286,  Feb.  2.  1903.     Nature,  69,  p.  463,  March  17,  1904.      • 
C.  G.  Barkla,  Roy.  Soc.  Phil.  Trans.,  204,  PP-  467-479,  May  31,  1905.   Roy.  Soc.  Proc.,  74,  pp.  474-475,  March  16,  1905. 
H.  Haga,  Konik,  Akad.  Wetensch.  Amsterdam  Versl.,  lo,  pp.  64-68,  July  20,  1906. 
J.  Herwig,  Ann.  d.  Phys.,  29,  2,  pp.  398-400,  May  21,  1909. 

E.  Basseler,  Ann.  d.  Phys.,  28,  4,  pp.  808-884,  Mar.  16,  1909. 

L.  Vegard,  Roy.  Soc.  Proc.,  Ser.  A.,  83,  pp.  379-393,  Mar.  22,  1910. 
W.  R.  Ham,  Phys.  Rev.,  xxx,  pp.  96-121,  Jan.,  1910. 

F.  C.  Miller,  Franklin  Inst.  Jour.,  171,  pp.  457-461,  May,  1911. 


276 


cidence  and  the  emergence  faces,  and  the  radiation  in  the  emergence  direction  is  much 
greater  in  intensity  than  that  in  the  incidence  direction.1 

Scattering  increases  with  the  thickness  and  with  the  atomic  weight  of  the  scattering 
substance,  and,  within  the  limits  ordinarily  used,  is  greater,  the  greater  the  penetrating 
ability  of  the  incident  rays. 

If  X-rays  of  very  low  penetrating  ability  are  allowed  to  fall  on  a  substance  (called  a 
"radiator"),  the  emergent  beam  contains  only  two  components  (a)  that  portion  of  the 
primary  beam  which  has  been  unaltered  and,  (b)  the  scattered  rays.  If,  now,  the 
penetrating  ability  of  the  incident  beam  is  gradually  increased,  a  point  is  at  last  reached 
at  which  a  new  type  of  radiation  appears,  which  is  characteristic  of  the  substance  used 
as  the  radiator.2 

Barkla  and  Sadler3  have  shown  that  if  the  incident  primary  rays  are  less  penetrat- 
ing than  are  the  secondary  rays  which  are  characteristic  of  the  radiator  used,  then  no 
secondary  rays  are  produced;  but  if  the  incident  primary  rays  are  more  penetrating 
than  the  characteristic  secondary  radiation,  then  "characteristic"  rays  are  produced. 


/oo  aoo  300 

Fig.  3 

The  production  of  characteristic  X-rays  is  very  analogous  to  the  production  of 
fluorescent  light.  In  fact,  the  analogy  is  so  close  that  some  writers  are  adopting  the 
term  "Fluorescence  Roentgen  Radiation." 

Chapman  and  Piper4  tried  to  detect  a  continuance  of  secondary  radiation  after  ex- 
citation from  the  primary  rays  had  ceased,  but  were  unable  to  detect  even  1/250  of  the 
original  radiation  1/3000  of  a  second  after  the  exciting  primary  rays  had  been  removed. 

Radiators  may  be  classified  into  four  groups  in  the  order  of  their  atomic  weights. 
The  radiations  given  out  by  the  members  of  each  group  are  very  much  alike. 

Group  1  (1-32}. -H-S.  When  excited  by  a  beam  from  a  "soft"  tube  the  members  of 
this  group  give  off  little,  if  any,  characteristic  radiation;  almost  the  entire  radiation  be- 
ing of  the  scattered  type  and  this  is  polarized  in  a  plane  perpendicular  to  the  direction  of 
the  parent  cathode  stream.  If  the  tube  is  made  moderately  "hard"  (i.e.,  if  it  gives  off 
rays  of  moderate  penetrating  power) ,  a  slight  amount  of  characteristic  radiation  will  be 
displayed,  and  if  the  tube  is  very  "hard,"  a  well-defined  characteristic  beam  is  given  off, 
having  a  penetrating  ability  much  different  from  that  of  the  exciting  rays. 

Group  2  (52-65}  .-Cr-Zn.  This  group  gives  off  a  beam  composed  almost  entirely  of  a 
true  characteristic  radiation,  even  when  excited  by  rays  from  a  "soft"  tube,  but  this 
radiation  has  little  penetrating  ability.  With  a  given  excitation,  the  ionization  pro- 
duced by  it  is  almost  100  times  that  produced  by  an  equal  mass  belonging  to  Group  1. 

1  Barkla  and  Ayers,  Phil.  Mag.,  pp. "270-280,  Feb.,  1911. 

Owen.  Proc.  Camb.  Phil.  Soc.,  vol.  16,  p.  161,  1911. 

Crowther,  Proc.  Roy.  Soc.,  pp.  478-494,  Feb.,  1912. 

t  C.  J.  Barkla  and  C.  A.  Sadler,  Nature,  77,  pp.  343-344,  Feb.  13,  1908,  Nature,  80.  p.  37,  March  11,  1919. 
3  Barkla  and  Sadler,  Nature,  80,  p.  37,  March  11,  1909. 
•>  Chapmen  and  Piper,  Phil.  Mag.,  19,  pp.  897-903,  June,  1910. 


277 


Group  3  (107-125). -Ag-I.  If  the  exciting  beam  is  only  of  moderate  penetrating 
ability,  this  group  gives  off  mostly  a  scattered  radiation,  but,  unlike  that  from  Group  1 , 
it  is  unpolarized,  and  there  is  a  noticeable  amount  of  characteristic  radiation  present. 
The  relative  amounts  of  scattered  and  characteristic  radiation  vary  greatly  with  small 
changes  in  the  character  of  the  exciting  rays. 

TABLE  II 

WAVE-LENGTHS  OF  VARIOUS  CHARACTERISTIC  X-RAYS 

NOTE. — The  most  important  line  in  each  series  is  called  a.     The  next  most  important  is  called  ft. 


Elements 

K  SERIES 

L  SERIES 

a 

0 

a 

ft 

X10-8  Cm. 

X10-8  Cm. 

X10-8  Cm. 

XIO-8  Cm. 

Al 
Si 
Ci 
K 
Ca 

8.364 
7.142 
4.750 
3.759 
3.368 

7.912 
6.729 

3^463 
3.094 

Ti 
V 
Cr 
Mn 
Fe 

2.758 
2.519 
2.301 
2.111 
1.946 

2.524 

2.297 
2.093 
1.818 
1.765 

Co 

Ni 
Cu 
Zn 
Y 

1.798 
1.662 
1.549 
1.445 
0.838 

1.629 
1.506 
1.402 
1.306 

- 

Zr 
Nb 
Mo 
Ru 
Rh 

0.794 
0.750 
0.721 
0.638 

6.091 
5.749 
5.423 
4.861 
4.622 

5^507 
5.187 
4.660 

Pd 
Ag 
Sn 
Sb 
La 

0.584 
0.560 

» 

4.385 
4.170 
3.619 
3.458 
2.676 

4.168 

3^245 
2.471 

Ce 
Pr 

Nd 
Sa 
Eu 

2.567 
2.471 
2.382 
2.208 
2.130 

2.360 
2.265 
2.175 
2.008 
1.925 

Gd 
Ho 
Er 
Ta 
W 

2.057 
1.914 
1.790 
1.525 
1.486 

1.853 
1.711 
1.591 
1.330 

Os 
Ir 
Pt 

Au 

1.397 
1.354 
1.316 
1.287 

1.201 
1.155 
1.121 
1.092 

Group  4  (1 83-206). W-Bi.    These  substances  resemble  Group  2  in  their  action. 

For  all  these  elements  the  penetrating  ability  of  the  characteristic  rays  is  inde- 
pendent of  the  intensity  or  of  the  penetrating  ability  of  the  exciting  beam,  but  is  a 
periodic  function  of  the  atomic  weights  of  the  radiating  elements.1 

If  the  radiator  is  a  chemical  compound,  the  component  atoms  and  radicals  deter- 
mine the  character  of  the  secondary  rays  produced.2 


C.  G.  Barkla  and  C.  A.  Sadler,  Phil.  Mag.,  16,  pp.  550-584,  Oct.,  1908. 
J.  A.  Crowther,  Phil.  Mag..  14,  PP-  653-675,  Nov..  1907. 


278 


The  rays  coming  from  salts  are  composed  of  (1),  a  homogeneous  radiation  having 
the  same  penetrating  ability  as  that  from  the  metal  itself,  and,  (2),  a  scattered  primary 
radiation  considerably  more  penetrating  than  that  of  (1)  due  to  the  acid  radical.  If  a 
metal  occurs  in  the  acid  radical  it  has  no  individual  effect,  but  merely  acts  with  the  re- 
mainder of  the  radical.1 

There  seems  to  be  a  real  physical  difference  between  primary  X-rays  and  char- 
acteristic X-rays.  From  the  electromagnetic  theory,  primary  rays  seem  to  consist  of  an 
irregular  succession  of  ''splashes."  Experimental  evidence  (which  will  be  taken  up  in  a 
latter  article)  seems  to  show  that  characteristic  rays  consist  of  trains  of  waves  resem- 
bling light  waves,  except  for  the  fact  that  the  wave  length  is  about  1/1000  that  of  or- 
dinary light.  Each  element  is  able  to  emit  characteristic  rays  whose  wave  lengths  fall 
into  certain  well  denned  groups.  Two  of  these  (called  K  and  L  respectively)  are  of 
great  importance.  For  any  given  radiator,  rays  of  the  K  group  are  about  300  times  as 
penetrating  as  those  of  the  L  group.  Table  II  shows  the  wave  lengths  of  the  two  most 
intense  members  of  each  group  for  39  elements.  It  will  be  noticed  that,  to  date,  there 

TABLE  III 

MINIMUM  SPEED  OF  CATHODE  RAYS  REQUIRED  TO  EXCITE  CHARACTERISTIC  RADIATIONS 


Radiator 

Atomic  Weight 
(0=16) 

Critical  Velocity  of 
Cathode  Rays  to 
Excite  K  Radiation 

Voltage  Necessary 
to  Give  Critical  Velocity 
to  Cathode  Rays 

Aluminum  

27.1 

cm.  /sec. 
2.06  X109 

volts 

1200 

Chromium  

52.0 

5.09X109 

7320 

Iron  

55.8 

5.83  X109 

9600 

Nickel  

58.7 

6.17X109 

10750 

Copper    . 

63.6 

6.26  X109 

11080 

Zinc             

65.4 

6  32  X  109 

11280 

Selenium 

79.2 

7  38  X  109 

15400 

are  many  gaps  still  to  be  filled  in  the  list.    The  values  given  in  the  table  were  published 
by  Mosely  in  the  Philosophical  Magazine,  April,  1914. 

Chapman  has  shown2  that  if  the  L  radiation  of  one  element  is  of  nearly  the  same 
wave-length  as  the  A'  radiation  of  another  element,  then  their  atomic  weights  (AL  and 
AK  respectively)  are  related  by  the  formula 


Whiddington  has  shown3  that  when  any  given  substance  is  used  as  the  target  of 
an  X-ray  tube,  it  will  give  off  characteristic  K  rays  if  the  speed  in  cm.  per  sec.  of  the 
cathode  rays  is  greater  than  108  times  the  atomic  weight  of  that  substance.  From 
Chapman's  formula  it  follows  that  the  characteristic  L  rays  would  be  obtained  at  a 
speed  of  ^  (A  —  48)  X  10*  cm.  per  sec.,  where  A  is  the  atomic  weight.  Table  III  shows 
Whiddington's  experimental  values. 

The  subject  of  electromagnetic  disturbances  was  discussed  at  the  beginning  of  this 
article  from  the  standpoint  of  a  single  retardation  of  the  electron.  It  is  evident  that,  if 
the  electron  had  been  considered  as  having  a  regular  to-and-fro  motion,  the  resulting 
electromagnetic  disturbance  would  have  consisted  of  a  train  of  waves  of  a  definite  wave 
length.  It  is,  therefore,  natural  to  assume  that  characteristic  X-rays  are  produced  when 
certain  electrons  in  an  atom  are  made  to  vibrate  at  some  natural  frequency.  Such 

1  J.  L.  Glasson,  Camb.  Phil.  Soc.,  pp.  437-441,  June  14.  1910. 

2  Chapman,  Proc.,  Roy.  Soc.,  1912. 

3  Whiddington,  Proc.  Roy.  Soc.,  1911. 

279 


vibrations  could  be  set  up  by  giving  the  electron  a  single  impulse,  provided  that  the 
time  consumed  in  communicating  the  impulse  is  not  more  than  half  the  time  of  one 
natural  vibration  of  the  electron.  We  thus  have  a  theoretical  basis  for  the  experimental 
facts  that  secondary  rays  may  be  produced  both  by  sufficiently  '  'thin"  primary  rays,  and 
also  by  cathode  rays  of  sufficiently  high  velocity.  It  is  plain  why  the  wave-length  of 
characteristic  rays  remains  the  same  when  the  breadth  of  the  primary  rays  is  decreased 
or  when  the  speed  of  the  cathode  stream  is  increased  beyond  the  critical  value,  and  why 
no  characteristic  rays  are  produced  until  these  critical  values  are  reached.  In  short,  the 
electromagnetic  theory  gives  us  a  rational  basis  upon  which  we  may  correlate  the  facts 
known  at  present  about  the  nature  of  X-rays. 


PART  II 

There  are  certain  general  properties  which  are  common  to  all  X-rays.  Among  these 
are: 

-  1.  The  fluorescent  effect 

-  2.  The  photographic  effect 

-  3.  The  ionizing  effect 

4.  The  chemical  effect 

5.  The  dehydrating  effect 

6.  The  photo-electric  effect 

7.  The  action  on  a  selenium  cell 

8.  The  penetrating  effect 

9.  The  physiological  effect 

(1)   The  Fluorescent  Effect 

Certain  uranium  compounds  and  certain  salts  of  the  alkali  and  alkali-earth  metals 
have  the  property  of  fluorescing,  i.e.,  of  giving  off  visible  light  when  exposed  to  the  ac- 
tion of  X-rays.  Many  of  these  compounds  phosphoresce,  i.e.,  continue  to  give  off 
light,  for  a  short  time  after  the  X-rays  have  been  cut  off.  In  order  to  be  of  great  use  in 
X-ray  work,  a  fluorescent  screen  should  possess  the  following  characteristics:  (a)  the 
color  of  the  light  should  be  such  as  to  give  good  visual  acuity;  (6)  the  intensity  of  light 
per  unit  intensity  of  X-rays  should  be  as  great  as  possible,  and  should  not  decrease  with 
continuous  operation  of  the  screen;  (c)  there  should  be  as  little  phosphorescence  as 
possible,  and  (d)  the  crystals  of  the  fluorescent  salt  must  be  so  small  that  the  "grain'' 
of  the  screen  is  not  visible  when  the  screen  is  in  use. 

The  selection  of  a  material  possessing  these  qualifications  is  a  long  tedious  task,  for 
the  fluorescent  properties  of  a  given  salt  are  greatly  changed  by  the  addition  of  minute 
amounts  of  impurities.  A  salt  showing  almost  no  fluorescence  when  pure  may  fluoresce 
brightly  when  mixed  with  but  a  fraction  of  one  per  cent  of  some  other  salt.  In  spite  of 
the  fact  that  each  formula  for  fluorescent  material  is  entirely  empirical,  there  are  three 
or  four  varieties  of  screens  on  the  market  which  meet  the  requirements  given  in  the  pre- 
ceding paragraph.  All  of  these  give  off  light  whose  intensity  is  determined  by  the  cur- 
rent through  the  X-ray  tube,  the  voltage  across  the  tube,  and  by  the  distance  from  the 
target  of  the  tube.  If  the  intensity  of  the  fluorescent  light  is  plotted  against  either  the 
current  or  the  voltage,  the  resulting  curve  is  a  straight  line  or  a  succession  of  straight 
lines.1  As  would  be  expected,  the  intensity  varies  inversely  as  the  square  of  the  dis- 
tance between  the  fluorescent  screen  and  the  target  of  the  tube. 


1  J.  S.  Shearer,  Am.  Journal  Rcentgenology,  Nov.,  1914. 

280 


(#)   The  Photographic  Effect 

X-rays  have  much  the  same  effect  on  a  photographic  plate  as  ordinary  light.  Just 
as  photographic  plates  are  more  sensitive  to  certain  wave-lengths  of  light  than  to  others ; 
in  the  same  way  they  are  found  to  be  more  sensitive  to  certain  wave-lengths  of  X-rays 
than  to  others.  This  is  usually  explained  by  assuming  that  the  plate  is  most  sensitive 
when  exposed  to  X-rays  of  such  wave-length  as  will  excite  characteristic  secondary  rays 
from  one  or  more  of  the  elements  in  the  emulsion.  Since  all  manufacturers  do  not 
necessarily  use  the  same  chemicals  in  making  their  X-ray  plates,  it  follows  that  (a)  all 
brands  of  plates  do  not  have  the  same  speed  when  excited  by  X-rays  of  the  same  wave 
length,  (6)  a  place  which  is  exceptionally  "slow"  when  excited  by  rays  of  long  wave 
length  (i.e.,  little  penetrating  ability)  may  be  quite  "  fast "  when  used  with  rays  of  short 
wave  length  (i.e.,  great  penetrating  ability). 

It  is  impossible  to  focus  X-rays  as  one  would  focus  visible  light  in  ordinary  pho- 
tography. Radiographs  are  merely  shadow  pictures.  If  all  parts  of  the  object  to  be 
radiographed  were  of  the  same  transparency  to  the  rays,1  all  that  one  could  possibly 
obtain  would  be  a  uniform  blackening  of  the  photographic  plate.  But.  if  one  part  of  the 
object  is  more  opaque  to  X-rays  than  some  other  part,  then  the  radiograph  will  show  a 
corresponding  change  in  density.  Thus  we  are  able  to  obtain  radiographs  of  the  human 
body  because  of  the  different  opacities  of  various  tissues,  and  it  is  possible  to  show  holes 
in  metal  castings  because  of  the  differences  caused  in  the  thickness  of  the  metal. 

In  ordinary  photography  the  chief  variables  to  be  considered  are  exposure  and  de- 
velopment. In  radiography  there  is  an  additional  variable,  namely,  penetration.  In 
choosing  the  proper  penetration,  the  radiographer  must  constantly  bear  in  mind  that  as 
the  penetrating  ability  of  the  X-rays  is  increased  there  is  an 
increased  tendency  to  blur  the  picture,  because  of  scattered 
rays.  It  is  also  necessary  to  remember  that  a  radiograph 
taken  with  very  penetrating  rays  is  less  "contrasty"  than 
one  taken  with  rays  of  moderate  penetration.  For  any 
given  amount  of  exposure,  the  optimum  penetration  is, 
therefore,  that  penetration  at  which  the  most  opaque  part 
of  the  object  permits  only  a  very  slight  darkening  of  the 
photographic  plate. 

Radiographs  are  always  examined  as  negatives.  A  radio- 
graph is,  therefore,  of  proper  density  when  it  can  be  easily 
viewed  against  a  clear  north  sky.  This  is,  as  a  rule,  much 
denser  than  is  advisable  for  making  the  best  positive  prints. 

(3)   The  Ionizing  Effect 

If  a  charged  body  is  exposed  to  X-rays,  it  will  be  found 
to  lose  its  charge.  This  is  explained  by  assuming  that  the 
air  in  the  path  of  the  X-rays  becomes  broken  up  into  ions,  or  electrically  charged  atoms. 
The  air  thus  becomes  a  conductor  of  electricity  and  causes  the  charge  to  leak  away. 
While  the  quantity  of  charge  carried  by  different  ions  is  not  always  the  same,  still  it  is 
always  some  exact  whole  number  times  4.9016X1CT10  electrostatic  units  of  charge.2 

Fig.  1  shows  an  ordinary  form  of  ionization  chamber.  Two  thin  sheets  of  aluminum 
foil  form  the  ends  of  a  metal  cylinder.  Midway  between  them  and  insulated  from  them 
is  another  thin  piece  of  aluminum  foil  This  middle  sheet  is  connected  to  a  quadrant 


'To  Electrometer 


To  Earth 


-F 


Fig.  1.     An  Ionizing  Chamber 


1  Transparency  to  X-rays  has  no  relation  to  transparency  to  ordinary  light.  An  object  may  be  quite  opaque  to  ordinary 
light  and  yet  be  very  transparent  to  X-rays  and  vice  versa. 

2R.  A.  Millikan,  Science,  23,  pp.  436-448,  Sept.  30, 1910.  This  is  the  strongest  evidence  we  have  that  electricity  has 
an  atomic  structure.  The  charge  on  an  electron  is  4.9016  X10~10  e.s.  units. 


281 


Sparking  / 


Current 


electrometer,  or  to  an  electroscope.  In  some  cases  it  is  possible  to  obtain  a  current  large 
enough  to  measure  with  a  galvanometer  connected  between  the  inner  and  outer  plates. 
This  type  of  ionization  chamber  has  many  advantages.  (Some  of  its  objectionable  fea- 
tures will  be  taken  up  later.) 

.  Suppose  such  an  ionization  chamber  to  be  exposed  to  the  action  of  X-rays  of  con- 
stant intensity  and  wave-lengths.  If  the  difference  of  potential  between  the  inner  and 
outer  sheets  is  small,  the  ionization  current  will  be  small.  As  this  difference  in  potential 
is  increased,  the  ionization  current  increases,  but  at  last  a  condition  of  "saturation"  is 

reached  in  which  the  current  is  inde- 
pendent of  the  voltage.  At  very  high 
potential  differences  the  current  once 
more  increases  and  sparking  soon 
occurs.  Fig.  2  shows  the  form  of  the 
voltage-current  curve  of  an  ionizing 
chamber. 

In  scientific  work  quantity  of  X-rays 
is  measured  in  terms  of  the  amount 
of  electricity  which  the  rays  are  able 
to  cause  to  flow  from  one  terminal  of 
the  ionizing  chamber  to  the  other. 
In  such  measurements  it  is  absolutely 
necessary  to  have  a  saturation  cur- 
rent through  the  ionizing  chamber, 

Fig.  2.     Voltage-Current  Curve  of  an  Ionization  Chamber  Otherwise    the    amount     of     electricity 

carried  across  will  depend  largely  upon 

the  potential  difference  between  the  plates  of  the  chamber.  It  is  also  necessary  to 
make  sure  that  the  chamber  is  of  proper  design  and  of  suitable  material. 

Owen,1  and  Barkla  and  Philpot2  have  shown  that,  except  when  characteristic  rays 
of  the  gas  were  strongly  excited,  ionization  in  that  gas  was  practically  independent  of 
the  wave-length  of  the  X-rays  employed.  Their  results  are  collected  in  Table  I. 
Methyl  Iodide  appears  to  be  an  exception  to  this  rule. 

In  a  previous  article  it  was  stated  that  when  X-rays  of  sufficient  penetrating  ability 
fell  upon  a  body,  that  body  became  a  source  of  characteristic  X-rays.  Under  such  con- 
ditions the  body  also  gives  off  a  large  number  of  electrons  (secondary  corpuscular 
radiation) .  These  electrons  have  the  ability  to  ionize  any  gas  in  which  they  find  them- 
selves. It  is  at  once  evident  that  there  are  serious  limitations  imposed  upon  an  ionizing 
chamber.  Before  the  amount  of  electricity  transferred  from  one  terminal  to  the  other 
may  be  taken  as  a  measure  of  the  quantity  of  X-rays,  it  is  necessary  to  make  sure  that 
the  material  used  in  its  construction  is  not  giving  off  secondary  corpuscular  radiation. 
It  is  equally  necessary  to  make  sure  that  the  gas  used  in  the  chamber  is  not  itself  giving 
off  secondary  corpuscular  radiation.  An  ideally  designed  chamber  is  so  constructed 
that  it  is  impossible  for  the  direct  X-ray  beam,  or  the  secondary  X-rays  scattered  by  the 
gas,  or  the  secondary  rays  characteristic  of  the  gas  to  strike  its  walls.  It  is  almost 
impossible  to  design  a  chamber  such  that  all  these  considions  will  be  fully  met  under 
all  circumstances.  Usually  the  design  is  made  such  as  to  be  quite  satisfactory  for  use 
with  the  wave-lengths  of  rays  that  are  to  be  measured. 


1  E.  A.  Owen,  Proc.  Roy.  Soc.  A.,  86,  pp.  426-439,  1912. 

2  Barkla  and  Philpot,  Phil.  Mag.,  xxv,  pp.  832-856,  1913. 


282 


In  many  cases  it  is  necessary  to  measure  the  total  amount  of  energy  in  the  X-ray 
beam.  This  may  be  done  by  using  an  ionization  chamber  long  enough  to  completely 
absorb  all  the  rays. 

C.  T.  R.  Wilson  has  shown1  that  it  is  possible  to  condense  water-vapor  upon  ions, 
thus  making  the  path  of  the  ions  visible.  Fig.  3  shows  the  arrangement  of  his  appara- 
tus. A  steel  ball  was  fastened  to  a  very  heavy  ball  by  a  thread.  The  heavy  ball  was 
hung  by  a  stout  cord.  When  the  cord  was  suddenly  loosened  the  ball  dropped  a  short 
distance,  opening  a  valve.  This  caused  the  bottom  of  the  expansion  chamber  to  be 
quickly  lowered  and  produced  a  condition  of  supersaturation  of  the  moisture  in  the  ex- 
pansion chamber.  The  stopping  of  the  large  ball  broke  the  thread,  thus  allowing  the 

TABLE  I 

RELATIVE  IONIZATION  PRODUCED  IN  VARIOUS  GASES  BY  HOMOGENEOUS  X-RAYS 


Element 
Emitting 
Characteristic 
K  Radiation 

IUNIZA1IUN    RELATIVE  TO  AIR  =1 

Ht 

(Beatty) 

02 

(B.&P.) 

CO-2 

(Owen) 

502 
(Owen) 

CiHzBr 
(B.&P.) 

CHSI 
(B.&P.) 

Fe                      0.00571 

1.37                 1.58 

11.3 

41.2 

Ni 

1.35 

1.55 

11.6 

162 

Cu 

0.00573 

1.38 

1.55 

11.8 

42 

152 

Zn                     0.00570 

1.42 

1.54 

11.5 

41.6 

.4s                     0.00573 

1.27 

1.51 

11.7 

42.2 

158 

Se 

1.31 

1.53 

11.8 

41.7 

Sr 

1.28 

1.53 

11.8 

153 

Mo 

1.28 

1.54 

11.5 

213 

188 

Ag 

1.32 

272 

198 

Sn 

0.04 

1.29 

335 

205 

Sb 

1.28 

I 

211 

Ba 

- 

251 

smaller  steel  ball  to  fall  freely.  In  its  descent  it  closed  a  spark-gap,  allowing  a  condenser 
discharge  to  pass  through  the  X-ray  tube.  After  a  predetermined  interval  it  closed  a 
second  spark-gap  which  allowed  another  condenser  discharge  to  pass  through  a  mer- 
cury vapor  lamp.  This  illuminated  the  cloud  in  the  expansion  chamber,  and  enabled 
the  paths  of  the  ions  to  be  photographed.  A  characteristic  photograph  is  shown  in  Fig. 
4.  As  a  result  of  his  work  Wilson  could  find  no  direct  action  of  the  X-ray  upon  the  gas. 
The  only  role  of  the  X-rays  seemed  to  be  that  of  causing  the  gas  to  give  off  electrons 
(i.e.,  ionize  the  gas).  These  electrons,  by  impact  upon  the  molecules  of  the  gas,  cause 
further  ionization. 

(4)   The  Chemical  Effect 

Except  for  the  action  of  X-rays  upon  the  materials  in  the  film  of  a  photographic 
plate,  the  only  chemical  actions  so  far  noticed  seem  to  be  a  precipitating  effect,  and  a 
hydrolytic  effect.  Iodine  is  precipitated  from  its  solution  in  chloroform  by  exposure  to 
the  rays.2 

Schwartz  has  found  an  ammonium  oxalate-mercury  bichloride  mixture  which  pre- 
cipitates calomel  under  the  action  of  X-rays.3 

When  starch  is  exposed  to  the  rays,  it  is  changed  into  soluble  starch  and  then  into 
dextrin.4 


1  Proc.  Roy.  Soc..  A.,  85,  p.  285,  1911. 

Proc.  Roy.  Soc.,  A.,  87,  pp.  277-299,  1912. 
*  H.  Bordier  and  J.  Galimard,  Arch.  d'Elect.  Med.  14,  Aug.  10,  Sept.  10,  1906. 

3  G.  Schwartz,  Wein  Med.  Presse,  xlvii.  2092,  1906. 

4  Colwell  and  Russ,   Arch.    Middlesex  Hosp.  xxvii,  p.  63.' 


283 


I  llur«i  n».rin  a    Sp*.rk 


(J)   The  Dehydrating  Effect 

Ammonium,  potassium,  barium,  and  magnesium  platinocyanides  change  color  when 
exposed  to  X-rays,  due  to  dehydration.1 

(6)   The  Photo-Electric  Effect 

As  was  stated  in  the  discussion  of  ionization,  when  a  body  gives  off  characteristic 
X-rays  it  also  gives  off  electrons.  These  electrons  leave  the  body  with  the  same 
velocity  that  a  cathode  stream  would  need  in  order  to  produce  the  X-rays  character- 
istic of  that  body.  This  is  true  no  matter  what  the  intensity  or  wave-length  of  the  ex- 
citing X-rays  may  be.  This  effect  corresponds  in  every  way  to  the  well  known  photo- 
electric effect  in  which  a  clean  metal 
surface  gives  off  electrons  when  illumi- 
nated by  ordinary  light. 

(?)     The  Action  Seleniiim  Cell 

X-rays  affect  the  electric  resistance 
of  a  selenium  cell  in  the  same  manner 
as  light.2 

(8)   The  Penetrating  Effect 

All  substances  exert  more  or  less 
of  an  absorbing  effect  on  X-rays.  In 
general,  the  absorbing  effect  of  any 
given  substance  for  a  given  bundle 
of  rays  depends  upon  the  material 
used  as  a  target  in  the  tube,  the  nature 
and  thickness  of  the  absorbing  sub- 
stance, the  history  of  the  radiation 
after  leaving  the  target,  and  the  poten- 
tial drop  across  the  tube  at  the  instant 
the  given  bundle  of  rays  is  given  off. 
Ordinarily,  if  the  target  in  the  tube  is 


Illuminating  Sp«.rK  G&p 


Fig.  3. 


Diagram  of  Wilson's  Apparatus  for  Photographing  the 
Paths  of  Ions 


a  substance  of  high  atomic  weight, 
then  rays  from  that  tube  will  be  less  readily  absorbed  than  if  the  target  had  been  of 
low  atomic  weight. 

When  rays  from  an  ordinary  X-ray  tube  pass  through  a  substance  some  of  the  radia- 
tion is  absorbed,  so  that  the  emergent  beam  acts  more  feebly  on  a  fluorescent  screen, 
photographic  plate,  selenium  cell,  or  ionizing  chamber.  If  rays  from  a  platinum-target 
tube,  operating  under  ordinary  working  conditions,  are  made  to  pass  through  silver, 
the  intensity  of  the  emergent  beam  may  be  calculated  from  the  formula, 


in  which 

/o  =  intensity  of  the  incident  beam. 

/  =  intensity  of  the  emergent  beam. 

e  =  base  of  natural  logarithms  =  2.  7  182. 

x  =  thickness  of  the  absorber. 

X  =  coefficient  of  absorption  of  the  substance  used  as  absorber. 


1  Bordier  and  Galimard,  Arch.  d'Elect,  Medical,  May  10,  1905. 
G.  Holzknecht,  Arch.  d'Elect.  Med.,  Oct.  10,  1910. 

2  Perreau,  Comtes  Rendus  129,  pp.  956-957,  Dec.  4,  1899. 
Athanasiadis,  Ann  d.  Phys.,  27,  4,  pp.  890-996.  Nov.  26,  1908. 


284 


X  is  very  approximately  independent  of  the  thickness  of  the  silver.1 

All  substances  for  which  the  above  law  holds  true  are  said  to  be  ''  aradiochroic."  If, 
however,  the  rays  are  made  to  pass  through  sheets  of  aluminum,  tin,  etc.,  the  above  law 
does  not  hold.  The  value  of-  X  as  calculated  by  the  formula  is  different  for  different 
thicknesses  of  the  absorbing  sheet;  but,  as  the  thickness  is  increased,  X  approaches  more 
and  more  nearly  to  a  constant  value,  and  the  law  holds  approximately  if  the  sheet  is 
thick  enough.  The  formula  may  also  be  made  to  apply  approximately  if  the  difference 
in  thickness  between  two  absorbing  sheets  is  so  small  that  the  value  of  X  has  not 
changed  appreciably,  due  to  the  change  in  thickness.  Absorbers  which  act  in  this  way 
are  said  to  be  "radiochroic,"  and  are  often  called  "niters." 

These  facts  may  be  explained  by  assuming  that  an  X-ray  tube  sends  out  a  complex 
radiation  ("heterogeneous  beam")  composed  of  primary  rays  and  secondary  rays 
characteristic  of  the  target.  In  aradiochroic  substances  the  coefficients  of  absorption 
for  the  component  beams  are  approximately  the  same.  In  radiochroic  substances  the 
beams  are  unequally  absorbed.  If  the  filter  is  thick  enough  to  completely  absorb  all 
but  one  of  the  components,  the  emergent  beam  is  said  to  be  "homogeneous,"  and  the 
absorption  law  will  hold  accurately  for  any  absorber  through  which  the  beam  may 
subsequently  pass,  provided  no  secondary  rays  characteristic  of  the  absorber  are  pro- 
duced. It  is  to  be  noticed  that  no  substance  is  absolutely  aradiochroic.  Silver  is  prob- 
ably the  most  nearly  aradiochroic  metal  for  the  rays  given  off  by  a  tube  with  a 
platinum  target;  but  it  is  quite  radiochroic  for  the  rays  given  off  by  a  lead  target,2  be- 
cause of  being  more  opaque  to  the  "secondary"  component  than  to  the  "primary" 
component  of  the  beam.  If  the  absorber  is  of  the  same  material  as  the  target,  it  is  more 
opaque  to  the  "primary"  component 
than  to  the  "secondary"  component; 
but  the  two  absorption  coefficients 
may  be  so  nearly  equal  as  to  cause  the 
absorber  to  appear  under  some  condi- 
tions to  be  almost  completely  arcdic- 
chroic.3 

If  the  absorber  is  of  the  same 
material  as  the  target,  then  X  decreases 
as  the  potential  difference  across  the 
tube  is  increased,  and  the  decrease  of  X 
with  the  increase  of  the  potential  dif- 
ference is  much  more  marked  the  higher 
the  potential  difference  employed.4 

If  the  absorber  is  a  chemical  com- 
pound, the  total  absorption  under  any 
specified  conditions  is  the  sum  of  the  various  absorptions  caused  under  those  conditions 
by  the  various  atoms  and  radicals  of  which  the  absorber  is  composed.5 

As  a  rough-and-ready  means  of  determining  the  penetrating  ability  of  X-rays,  physi- 
cians use  what  they  call  "penetrometers."  Of  these  the  Benoist  and  the  Whenelt  are 
the  most  accurate.  In  both  of  these  instruments  the  opacity  of  a  standard  sheet  of 

1  L.  Benotis,  Journal  de  Physique,  1901. 

J.  Beloit,  Arch  d'Elect.  Med.,  Aug.  10,  1910. 

-'  W.  R.  Ham,  Phys.  Rev.  xxx,  1,  Jan.  1910,  pp.  104-105,  118-120. 
3  G.  W.  C.  Kaye,  Camb.  Phil.  Soc.  Proc.  14,  pp.  236-245,  Oct.  15.  1907. 

Roy.  Soc.  Phil.  Trans.  A,  209,  pp.  123-151,  Nov.  19,  L908. 
'  W.  R.  Ham,  Phys.  Rev.  xxx,  1,  pp.  108,  111-113,  Jan.,  1910. 
•'  W.  Seitz.  Phys.  Zeitschr.  13,  pp.  476-480,  June  1,  1912. 

Blennard  and  Labesse,  Comtes  Rendus,  1896,  cxxii.  pp.  723-725. 


• 


. 


! 


Fig.  4.     Photograph  of  Paths  of  Ions  Taken  by  C.  T.  R.  Wilson 


285 


silver  is  matched  against  the  opacity  of  aluminum  of  various  thicknesses.  In  the  case  of 
the  Whenelt  penetrometer,  a  standard  piece  of  silver  is  fastened  to  a  fluorescent  screen. 
A  specially  shaped  wedge  is  slid  past  the  screen  until  a  thickness  is  at  last  reached  such 
that  the  illumination  of  the  screen  under  the  aluminum  is  the  same  as  that  under  the 
silver.  The  penetrating  ability  or  "hardness  "  of  the  rays  is  read  on  an  arbitrary  scale. 
The  Benoist  penetrometer  consists  of  a  disk  of  silver  0.11  mm.  thick.  Around  this 
disk,  arranged  like  a  circular  staircase,  are  steps  of  aluminum,  each  step  being  one 
millimeter  higher  than  the  one  preceding.  A  radiograph  of  the  penetrometer  is  taken 
and  the  "hardness"  is  defined  as  the  number  of  millimeters  of  aluminum  which  are  as 
opaque  as  the  disk  of  silver.  Care  must  be  taken  not  to  over-expose  the  radiograph, 
otherwise  the  whole  negative  will  be  blurred  and  a  correct  reading  will  be  found  im- 
possible. 

(9)   The  Physiological  Effect 

When  X-rays  are  directed  toward  a  given  layer  of  flesh,  in  general,  some  of  them 
pass  though  while  others  are  absorbed  and  give  up  their  energy  to  the  flesh.  If  suffi- 
cient energy  is  thus  delivered  to  the  flesh,  serious  pathological  changes  result  which  are 
of  great  importance  from  the  viewpoint  of  the  physician,  but  which  do  not  concern  the 
physical  investigator,  aside  from  the  question  of  his  own  self -protection.  A  person  in 
good  health  may  have  several  radiographs  taken  (sufficient  for  ordinary  diagnostic 
work)  by  a  well-informed  operator,  without  any  danger  of  an  X-ray  burn ;  but  the  oper- 
ator, or  research-physicist,  must,  because  of  the  possibility  of  long-continued  exposure 
or  more  often  because  of  frequent  repetition  of  short  exposures,  protect  himself  most 
carefully.  The  German  Roentgen  Society  recommends  that  for  work  such  as  is  or- 
dinarily done  by  physicians  the  protection  should  consist  of  at  least  two  millimeters  of 
sheet  lead,  eight  millimeters  of  X-ray  proof  rubber  impregnated  with  lead,  or  lead 
glass  from  ten  to  twentv  millimeters  thick.1 


1  Archives  of  the  Roentgen  Ray,  July,  1913. 

PART  III 

In  Part  I  of  this  series  it  was  shown  that  the  electromagnetic  theory  of  X-rays  con- 
siders characteristic  X-rays  as  being  light  of  very  short  wave  length.  It  would  be  rea- 
sonable to  expect  that,  if  this  were  so,  characteristic  X-rays  would  obey  the  same  laws 
of  diffraction  as  light  waves.  The  experimental  verification  of  this  expectation  is  one 
of  the  most  signal  vindications  of  the  electromagnetic  theory. 

Before  considering  the  experiments  themselves  it  will  be  well  to  consider  the  dif- 
fraction of  ordinary  light.  Let  XX1  (Figs.  1,  2  and  3)  be  a  surface  opaque  to  ordinary 
light,  and  let  B  and  E  be  narrow  slits.  If  the  two  rays  of  light  AB  and  DE  come  origi- 
nally from  the  the  same  portion  of  the  same  source  by  paths  of  equal  length,  then  the 
light-wave  at  B  will  be  in  phase  with  the  light-wave  at  E.  Therefore,  B  and  E  will  act 
as  exactly  similar  sources  of  light.  A  lens  placed  at  CF  will  cause  the  light  to  appear  at 
the  focus  as  a  luminous  spot,  for  the  two  beams  of  light  will  meet  in  the  same  phase 
having  traveled  equal  paths  from  B  and  E.  If  the  lens  is  moved  slightly  to  one  side  the 
intensity  of  the  light  will  be  much  diminished,  for  the  light  from  B  and  E  will  travel 
over  paths  of  different  lengths.  If  the  lens  is  moved  further  to  one  side,  no  light  will  be 
seen  at  the  focal  spot.  But  if  the  lens  is  moved  to  a  position  GH  (Fig.  1)  such  that  the 
difference  in  path-length  is  one  wave-length,  then  the  two  beams  once  more  meet  in 
phase  and  light  is  seen.  In  the  same  way  light  may  be  observed  at  //  and  KV  (Figs. 
2  and  3)  where  the  difference  in  path-length  is  two  and  three  wave-lengths  respectively. 

286 


The  light  appearing  at  CF  is  usually  called 
an  image  of  the  "zero  order,"  that  at  GH 
an  image  of  the  "first  order,"  etc.  The 
intensity  of  the  light  falls  off  rapidly  as  the 
"order"  of  the  image  increases,  so  that  only 
the  first  few  "orders"  can  be  made  use  of. 
An  inspection  of  Figs.  1,  2  and  3  will 
make  it  at  once  evident  that,  if  the  angles 
separating  the  various  orders  of  images  are 
to  be  large  enough  to  be  measurable  with 
accuracy,  the  distance  d  must  be  compar- 
able with  the  wave-length  of  the  light  used. 
Thus  diffraction  gratings  intended  for  use 


Fig.  1.     Diagram  Showing  Production  of  a  Spectrum 
of  the  1st  Order 

hope  of  overcoming  this  difficulty  but  they 
did  not  achieve  much  success. 


Fig.  2. 


Diagram  Showing  Production  of  a  Spectrum 
of  the  2nd  Order 


with  visible  light  (wave-lengths  in  the 
neighborhood  of  10~5  centimeters)  usually 
have  a  grating  space  d  of  from  0.002  cm.  to 
0.0005  cm.  Now  the  wave-length  of  X-rays 
is  in  the  neighborhood  of  10~6  centimeters. 
It  is  evidently  hopeless  to  attempt  to  rule 
a  grating  so  fine  that  the  lines  are  only  one 
hundred-millionth  of  a  centimeter  apart. 

Haga  and  Wind1  and  Walter  and  Pohl2 
tried  to  use  a  single  narrow  wedge  in  the 


Fig.  3. 


1  Haga  and  Wind,  Wied,  Ann.,  pp.  884-895,  1899. 
'  Walter  and  Pohl.  Ann.  d.  Phys.,  pp.  715-724,  1908. 


Diagram  Showing  Production  of  a  Spectrum 
of  the  3rd  Order 


287 


To  Prof.  Laue  of  Munich  belongs  the  credit  for  the  next  great  step  in  the  dif- 
fraction of  X-rays.  The  atoms  in  a  crystal  are  arranged  in  a  definite  systematic  forma- 
tion and  their  inter-atomic  distances  are  of  the  same  general  order  of  magnitude  as  the 
wave-lengths  of  X-rays,  as  calculated  from  theory.  Laue,  therefore,  was  led  to  regard 
a  crystal  as  a  ready-made  natural  diffraction  grating  for  use  with  X-rays.  Such  grat- 
ings are  much  more  complicated  than  those  used  with  ordinary  light  because  of  their 
three-dimensional  nature.  Friedrich  and  Knipping1.  verified  experimentally  Laues' 
conjecture.  Their  apparatus  is  shown  diagrammatically  in  Fig.  4,  and  Fig.  5  shows 
diffraction  patterns  which  they  obtained. 

It  will  be  noticed  from  Fig.  4  that  Friedrich  and  Knipping  used  the  crystal  as  a  trans- 
mission grating.  The  method  was  capable  of  showing  that  X-rays  could  be  diffracted, 
but  was  not  well  adapted  to  measuring  the  wave-length  of  X-rays  nor  to  a  study  of 
crystal  structure.  W.  L.  Bragg  conceived  the  idea  of  using  a  crystal  as  a  reflection  grat- 
ing (see  Fig.  6).  This  method  possesses  the  advantage  that  the  results  are  very  easy  of 
interpretation,  which  is  evident  at  once  by  an  inspection  of  Fig.  7. 

The  distance  MNP  is  the  difference  in  path-length  of  the  two  X-ray  beams  AI  and 
A2.  If  this  distance  is  an  exact  whole  number  of  wave-lengths,  the  two  reflected  waves 
are  in  phase  and  the  wave  actually  exists.  Otherwise,  the  waves  are  out  of  phase  and 

interfere  destructively.   There  are  thus  only 
,  ,,.„  two  conditions  to  be  met,  (a)  the  angle  of 

L  a.  3a     \j  i  &  pn  r*a  o  rr» ,5 

reflection  must  equal  the  angle  of  incidence, 
ph,c  (b)  the  wave-length  of  the  X-rays  must  be 
such  that  it  is  an  exact  divisor  of  the  differ- 
ence in  the  path-length  caused  by  reflection 
from  the  various  crystal  planes.  If  the 
inter-atomic  distances  and  the  angles 
between  crystal  planes  are  known,  the 
wave-length  of  any  X-ray  beam  may  be 
calculated  from  the  angle  at  which  it  will 
reflect.  Conversely,  if  the  wave-length  cf 
the  rays  and  the  angle  at  which  they 
are  known,  then  from  the  angles  between  the 


1  Sen 
•^ 

«n. 

J/ 

/  ryi 

i 

« 

'l 

l 

x-PhotcO'-<3p 
'     Plate 

! 

i 

r   • 

J^^\ 

• 

Sightrng 

Fig.  4. 


Diagram  of  Apparatus  Used  by  Friedrich  Knipping 
and  Laue 


reflect   from  a  given    crystal   face 

crystal  planes  the  inter-atomic  distances  may  be  calculated. 

An  attempt  has  been  made  in  Fig.  7  to  show  by  lines  drawn  through  the  atoms  that 
there  are  many  crystal-planes  possible  in  different  directions  in  a  single  crystal.  An 
inspection  of  the  diagram  shows  at  once  that  the  inter-atomic  distance  is  different  for 
different  planes.  By  examining  the  various  orders  of  spectra  from  the  various  crystal 
planes,  Prof.  Bragg  has  been  able  to  assign  a  definite  arrangement  to  the  atoms'  of  a 
crystal.  Fig.  S  shows  two  spectra  obtained  by  reflection.  Fig.  9  shows  the  structure-of 
a  crystal  of  a  halogen  salt  of  a  monovalent  metal.  The  black  spots  represent  metallic 
atoms,  (Na,  K,  etc.).  The  white  spots  represent  the  halogen  (F.  CL,  Br,  I). 

As  a  result  of  the  work  in  X-ray  spectra,  the  Mendelejeff  table  of  elements  is  to  be 
largely  replaced  by  the  Rutherford  system  of  atomic  numbers  (Table  I)  which  is  based 
on  the  fact  that,  if  the  elements  are  arranged  in  the  order  of  their  atomic  weights,  the 
atomic  numbers  are  proportional  to  the  reciprocal  of  the  square  roots  of  the  wave- 
lengths of  the  characteristic  X-rays. 


1  Friedrich,  Knipping  and  Laue,  Ann.  d.  Phys.,  pp.  971-1002,  1913. 


288 


Fig.  5a.     Through  Crystal  of  Zinc  Blend. 

Rays    Passed    Through    Crystal 

Perpendicular  to  100  Plane 


r  O.OS77 
i  O,  off 3 
0,  Of  (3 
o,  lost 


w    +  0 

o°©     '    + 
o© 


o© 


Fig.  5b.     Theoretical  Diagram    Showing 
Position  of  Possible  Spots  on  Fig.  5a 


Fig.  5c.     Through  Crystal  of  Zinc  Blend. 

Rays  Passed  Through  Crystal 

Perpendicular  to  1 1 1  Plane 


Fig.  5d.     Same  as  Fig.  5c  Except  that  the 
Crystal  has  been  Slightly  Rotated 


Fig.  5e.     Through  Copper  Sulphate. 

Rays  Passed  Perpendicular 

to  1 10  Plane 


Fig.  5f.     Same  as  Fig.  5e,  except  that 

Photographic  Plate  is  Farther 

from  Crystal 

Fig.  5.     Spectra  Obtained  by  Knipping  and  Laue 
289 


Fig.  5g.     Rays  Passed  Through 
Powdered  Copper  Sulphate 


A  striking  instance  of  the  interrelation  of  the  various  branches  of  science  is  furnished 
by  the  work  of  the  physicist  in  X-rays  which  has  been  found  useful  not  only  in  physics 


Fig.  6.     Arrangement  of  Apparatus  Used  by  Bragg 


Ca 


Cu. 


Bro.53 


Increasing  V\/a 


Fig.  7.     Diagram  Showing  Reflection  by  Atoms  in  a  Crystal          Fig.  10.     Spectra  Obtained  Using  Various  Metals  as  Reflectors 

but  also  in  crystallography  and  chemistry.    The  crystallographer  is  now  able  to  meas- 
ure in  centimeters  the  distances  between  the  molecules  in  crystals  and  is  able  in  many 


1st  Order 


Rhodium  Ant/cathode 
Rock --Salt  Reflector 


ZndOrder 


1 


i 


5°  10°  IS9 

Glancing  Angles  of  Incidence  of  X 


Platinum  dnticathode 
flock-Salt  Kef  lector 


5°  .   10°     IS"     20°    25°     30°      35°  40° 
•, a 'ncmg  Anqlea  of  Incidence  of  X  Rays 


Fig.  8.     Spectra  Obtained  by  Bragg 


cases  to  assign  a  definite  structure  to  them.    The  chemist  now  knows  that,  at  least  as 
far  as  crystals  are  concerned,  there  are  no  such  things  as  molecules  in  the  sense  in  which 


290 


TABLE   I 


THE  RUTHERFORD  SYSTEM  OF  ATOMIC  NUMBERS 


Atomic  Numbers 

Symbol 

Atomic  Weight 

Atomic  Numbers 

Symbol 

Atomic  Weight 

1 

H 

1.008 

57 

La 

139. 

2 

He 

3.99 

58 

Ce 

140.25 

3 

Li 

6.94 

59 

Pr 

140.6 

4~ 

Gl 

9.1 

60 

Nd 

144.3 

5 

B 

11. 

61 

.     6 

C 

12. 

62 

Sa 

150.4 

rr 

N 

14.01 

63 

Eu 

152. 

8 

O 

16. 

64 

Gd                        157.3 

9 

F 

19. 

65 

Tb                        159.2 

10 

Ne 

20.2 

66 

Ho 

163.5 

11 

Na 

23. 

67 

Dy 

162.5 

12 

Mg 

24.32 

68 

Er 

167.7 

13 

Al 

27.1 

69 

Tmi 

14 

Si 

28.3 

70 

Tmu 

15 

P 

31.04 

71 

Yb 

172. 

16 

S 

32.07 

72 

Lu 

174. 

17 

Cl 

35.46 

73 

Ta 

181.5 

18  ' 

A 

39.88 

74 

W 

184. 

19 

K 

39.1 

75 

j  - 

20 

Ca 

40.07 

76 

Os 

190.9 

21 

Sc 

44.1 

77 

Ir 

193.1 

22 

Ti 

48.1 

78 

Pt 

195.2 

23 

V 

51. 

79 

Au                        197.2 

24 

Cr 

52. 

80 

'        Hg 

200.6 

25 

Mn 

54.93 

81 

Tl 

204. 

26 

Ft 

55.84 

81 

RaC2 

210. 

27 

Co 

58.97 

81 

AcD 

28 

Ni 

58.68 

81 

ThD 

208. 

29 

Cu 

63.57 

82 

Pb                       207.1 

30 

Zn 

65.37 

82 

RaB 

214. 

31 

Ga 

69.9 

82 

RaD 

210. 

32 

Ge 

72.5 

82 

ThB                     212. 

33 

As 

74.96 

82 

AcB 

34 

Se 

79.2 

83 

Bi                        208. 

35 

Br 

79.92 

83 

RaC 

214. 

36 

Kr 

82.92 

83 

RaE 

210. 

37 

Rb 

85.45 

83 

AcC 

38 

Sr 

87.63 

83 

ThC 

212. 

39 

Yt 

89. 

84 

RaA 

218. 

40 

Zr 

90.6 

84 

RF 

210. 

41 

Nb 

93.5 

85 

42 

Mo 

96. 

86 

Nt 

222. 

43 

87 

44 

Ru 

101.7 

88 

Ra 

226. 

45     . 

Rh 

102.9 

88 

Mes.  Thi 

228. 

46 

Pd 

106.7 

88 

AcX 

47 

Ag 

107.88 

88 

ThX 

224. 

48 

Cd 

112.4 

89 

Ac 

49 

In 

114.8 

89 

Mes.Thu            228. 

50 

Sn 

119. 

90 

Th                        232. 

51 

Sb 

120.2 

90 

Ux 

230.5 

52 

Te 

127.5 

90 

lo 

230.5 

53 

I 

126.92 

90 

RaAc 

54 

•    Xe 

130.2 

90      , 

Rath 

228. 

55 

Cs 

132.81 

91 

Ux* 

'  *•                                           * 

56 

Ba 

137.37 

92 

U 

238.5 

291 


the  word  is  ordinarily  used  in  chemistry.    The  whole  crystal  is  a  big  complex  molecule, 
and  the  chemical  formula  only  shows  the  relative  amounts  of  each  element  present.    It 

£ Q £  is  reasonable  to  assume  that  in  metals  the  crystal  (not 

(>— nfih- -T)  the   "molecule")  is,  next  to  the  atom,  the  unit  to  be 

^        '    r-O        I    I  •       considered.    It  has  even  been  suggested  that  it  might 

be  possible  to  use  the  characteristic  X-rays  as  a  means 
of  identifying  the  various  elements,  so  that  a  measure- 
ment of  the  wave-length  of  the  characteristic  X-rays 
from  a  substance  would  serve  as  a  qualitative  analysis 
of  the  substance.    Fig.  10  shows  in  diagram,  spectra  of 
the  third  order   obtained    by  Mosley.*    They    are    so 
Fig.  9.    Arrangement  of  Atoms  in  a       arranged  that  the  scale  of  wave-lengths  of  each  plate 
crystal  of  Sodium  chioiide  registers  with  the  scale  of  the  plates  above  and  below 

it.    Table  II  which  is  of  approximate  wave-lengths  is  given  for  reference. 


*  Mosley,  Phil.  Mag.,  Dec.,  pp.  1024-1034,  1913. 

TABLE  II 
VARIOUSELEMENTS,  THEIR  ATOMIC  WEIGHTS,  AND  WAVE-LENGTHS  OF  THEIR  CHARACTERISTIC  X-RAYS 


Element 

Atomic  Weight 

Wave-  Length 

Remarks 

Calcium.  

40:1 

3.36    X10~8cm. 

Strong  K  radiation 

Titanium  

48.1 

3.09    X  ID"8  cm. 
2.76    X1Q-8  cm. 

Weak         radiation 
Strong  K  radiation 

Vanadium  

51.1 

2.525X10-8  cm. 
2.52    X  ID"8  cm. 

Weak         radiation 
Strong  K  radiation 

Chromium  

52.0 

2.30    X10~8  cm. 
2.30    XlO-8cm. 

Weak         radiation 
Strong  K  radiation 

Manganese  

54.9 

2.09    X  10~8  cm. 
2.11    X10~8cm. 

Weak         radiation 
Strong  K  radiation 

Iron                                        i 

55.9 

1.92    X  ID"8  cm. 
1.945  X10~8  cm. 

Weak         radiation 
Strong  K  radiation 

Cobalt  

59.0 

1.765X10-8  cm. 
1.80    X  ID"8  cm. 

Weak         radiation 
Strong  K  radiation 

Nickel   

58.7 

1.63    X  ID"8  cm. 
1.66    XlQ-8cm. 

Weak         radiation 
Strong  K  radiation 

Copper.  . 

63.6 

1.505X10-  [cm. 
1.55    XlO-jcm. 

Weak         radiation 
Strong  K  radiation 

1.40    X1Q-,,  cm. 

Weak         radiation 

292 


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out  IT 

DEC  12  194 

> 

• 

JUN   121043 

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^x- 

LD  21-100m-7'33 

